Things happen, and when you are trying to keep track of 152 tiles (if you play by NMJL rules) or 144, or even 136 (all used for different types of play), tiles can get lost or damaged. I have heard stories of dogs eating old Bone and Bamboo tiles (no surprise: part of the tile is bone, and perhaps the older the tastier?!), tiles falling on the floor and breaking apart, or cases flying open with tiles strewing everywhere.
Help is available, and here are some of my favorite resources.
"CHarli" has a fabulous website. I think my first visit there whetted my appetite for collecting. When on her site, be sure to browse through the Museum and Book pages since she has a lot of very helpful information. If you are looking to get tiles to use for jokers, she has a great selection. You will have to send her one of yours-there are so many variations in color and shape this is the only way she feels she can be helpful. She also sells wonderful joker stickers.
Another website is run by Matthew Shim. His site is different: you can look for exact tiles you may be missing. He shows what specific tiles he has in stock, and you can buy them by sending him an email, and he will bill you through paypal. If the tiles don't match yours, you can return them for a full refund within 30 days, minus his shipping fee. Be sure to look carefully at all the tile sets; sometimes you may find what you are looking under another Manufacturer's name.
You may have extra tiles but you may be missing one or two tiles. People have been known to make their own stickers, placing a mate to the missing tile in the scanner and using paper with a sticky back to use as a sticker on one of the extra tiles. Some people claim they can't even tell the difference! I have heard the white waterproof vinyl by Papilio works well.
Sometimes tiles lose their paint and need to be repainted. A wonderful person for this work is Dee Gallo. She is skilled at carving too, so if you have a blank tile that can be carved to replace your missing or damaged tile, she can do it. Dee designs beautiful limited edition sets, so be sure to look at her whole website.