Instructions
Hunt for loose Lego wheels at garage sales, flea markets or thrift stores. If you are patient, this is the cheapest approach. Look at classified ads and search websites like Craigs List, Toy Brick Brigade or iBrick Shop for new and used wheels.
Shop at online school supply stores. My Child Playhouses sells a Lego education set that includes two different sizes of tires, plus axles and wheel hubs. Discount School Supply sells a larger educational set of 360 pieces, including wheel elements; colors are limited in black, gray and white.
Go to Isodomos.com to match tires to the appropriate rim. This website links to various other sites and databases.
Purchase used Lego wheels from Lego user groups such as Lugnut. Try LegoSwap.com, an online community that swaps used Lego parts. Enrollment is free for parents and kids.
Go to Legoland in California, London or Germany. The parks offer the largest Lego selection anywhere. Go to Lego Clubhouse in Fun Town to stock up on wheels.