There’s a few different ways to do it. My local hobby store sells decal sets for models and R/C cars–just the decals. I look through them, trying to find something close to the right size, cut them out, and then just follow the standard wet-transfer procedure for adding them on. Specifically, I’ve been using NASCAR decals, and cutting out the smaller sponsor logos. I’ve also used just plain old transfer lettering from an art store for “custom” lettering (like the “BXU” I’ve been putting on most of my cars).
The other way to do it is to print your own decals. I just pointed Derek towards some clear decal sheets that can be used with color inkjet or laserjet printers. The decals are wet-transfer, so they need to have a sealer applied to them after printing and before using. The kind I have are from BARE-METAL FOIL & HOBBY CO. The weblinks shoud work. If not, it’s http://www.bare-metal.com. The clear sheets I have are part number 124. Just under $3.00 USD for a letter-sized sheet.
They also make a plain-old clear sticker sheet for use with color inkjets and laserjets, but I haven’t used those. Wet transfers and dry-transfers look so much better when applied.
And for getting the logos for those do-it-yourself sticker sheets, your best resource is the web. Save logos to your hard-drive and use whatever image software you have (even Windows Paint–my personal favorite) to manipulate the images to the right size for the car. HobbyLink Japan (http://www.hlj.com) even carries pictures of some of the decal sheets they sell–a fantastic resource I’m using for my WRX project.