I think on the Lego website, they have a form you can fill out for missing pieces. It’s not a guarantee that you’ll get them, but worth a shot.

Bricklink. Look up each set by number, go to parts list and verify each part. If you are missing one, add it to a wanted list, then shop for parts from your wanted list. You will likely need to order from many different stores. I start with a rare piece, find someone selling it for a fair price and see what else off of my list they have.

Give Lego customer support a call, the phone number is in the instruction guide or on their website and they can send you the missing pieces.

Most likely you lost them. Very rarely does it actually happen in my experience

Always check the corners of the piece bags and check inside larger pieces.

It is fairly rare that sets are missing pieces and I almost always find the other piece in the bag, the box, or tucked inside the back of another piece.

As already mentioned, customer support will replace any pieces you might be missing if it truly is missing.

If you're ever looking for pieces in general, you can check online retailers for parts or local stores. I use bricklink, but there are probably other secondhand retailers. Lego.com has an ever-broadening selection of pick-a-brick options as well.

Also, check the plastic bag to ensure you got out all of the pieces before throwing it away (I've made this mistake). But if you check what the first commenter said, and then make sure you didn't throw them away, you can file a request for missing parts on the lego website and they'll ship them to you for free. It usually takes 7-10 days.

Check your floor with a fine tooth comb. If all else fails, contact LEGO customer service.

Agree with others. 99% of the time I've either overlooked it, laid it some place weird, or the piece made a break for it and it's somewhere on the floor.

Did you use them when you were supposed to use the 2L technic beams with pin and axle holes?

If you can't find them anywhere then go to https://www.lego.com/en-us/service/replacementparts

BrickLink.com

Bricklink or eBay. I’ve had good luck from eBay with people selling parts. Cost me £2 including delivery for 6 of a part I needed

No contact Lego customer support and they'll send you a replacement piece they're really good about that kind of thing.

Double, tripplw check and then check again all th bags, check the floor, check under the seat. The number of times I've got mad about a missing piece only to find it later.

That said, it does happen and you can get a replacement from Lego via their support section on the website.

Start a missing parts request

OP 2 hours later

Edit: found it under my couch!

You can contact Lego and they’ll send you the missing piece for free. It’s pretty fast, iirc.

no reason not to try lego broken part service regardless of age. it's a very simple process and will let you know if they will cover it or not. also depending on the part...i'm not saying you should do this...but you can just pick a newer set and get the piece from that one if it's too old.

i've gotten replacement parts of the legendary brittle brown from my haunted house from 2012. just placed an order for replacement spiderman legs from 2016 yesterday

Bricklink is the way to go. Parts for those sets likely aren't in production any more. Lego will have updated colors and new mold variants.

I wouldn't use the missing service for parts that have gone missing from my old sets. The intent there is to replace parts that are missing from the original packaging when you buy a set. I would use Bricklink for those.

For broken parts, Lego can only replace parts that they still make and have in stock. For older sets, they are less likely to have parts than Bricklink. I'd check, but probably just use Bricklink because it's faster.

Can you? Sure. Should you? No.

Legos have some incredible precision and 3D printers can't even get close to that precision meaning that the parts will either have too much or too little clutch power. And dialing in the dimensions will be a painful process too. You might as well just get some free replacements like u/ramk13 said or buy replacements through a site like bricklink.

The only good use for printed Legos is if you're making a custom part that doesn't exist in the actual Lego inventory.

You mean generic interlocking plastic construction bricks? Because people only print generic interlocking plastic construction bricks, we would NEVER print Lego that would be bad and wrong. 

This is not what you are asking, but Lego will send you replacements for missing pieces for free. They have a web form you can fill out and the pieces show up in a few weeks. Over hundreds of sets I've only done it twice, and it wasn't for missing pieces, it was for broken/mangled ones.

There is no automated way but you certainly could do this on Bricklink. Create a wanted list then populate the list with the info from your spreadsheet. You will end up with a list of all your missing pieces including a picture and the number of each missing pieces. In the remarks section of each item you can indicate in which set it is missing. If the same part is missing in multiple sets you can also indicate in the remarks section that 2 go to this set, 1 for this set and so on.

I use a small notebook . set number then parts discription - 4x2 flat brown . I find it easy to pick up and flick thro' anywhere anytime. Also satifiying to cross out when I find a replacement. I do this with every second hand set I get . I find ts quicker.

I had my first missing piece in almost 30 years of building and over 500 sets just last month.

I think the 100 odd sets I’ve bought, only one have had a missing piece.

I think to get multiple does lean towards user error. Did you have any similar size left over parts after finishing each set?

And have these sets been in the new paper bags? Since they are crazy good at keeping pieces hidden inside. I went mad about 5 times while building the UCS Sail Barge.

Yep, 30+ years collecting…at least 1 set per month…never had more than 1 missing piece before 5 years ago…now it’s 1 in every 3-4 sets at least 1 common piece is missing

At least once a year I have a set with a close to dozen pieces either missing or wrong colour

As a serious collector I have a big stockpile of spares and I can normally overcome it…but these days they always seem to want to have a more unique element or colour, and replacements are not always on hand

The worst I have ever seen was from the Home Alone House. Almost 35 missing pieces....and I had to go through hoops to get them....had to send 3 photos, payment receipts, the lot number from the box, a total of 8 emails and then it took 2 months to get them.

I agree....quality control is down hill

I was missing two of the SAME pieces from the home alone set that I just bought.