James's review of Transfer Stickers
I went with carstickers.com because they offered the lowest price for the type of sticker that I wanted to make. The end result came out looking fantastic and there haven't been any problems with it even though I wasn't able to leave the transfer paper on it for more than a couple of hours. The only complaint I have is that the upload instructions didn't make it clear that any white parts of the image would be removed -- I had to redo my design because of that.