Shadow Ivy
jungleguu is correct. You can just get inkjet-friendly sticker paper at any office supply store, computer store, or even Target. Cutting them out yourself is definitely rather tedious, but it's something you could do while, say, watching TV or something, so that way it doesn't seem so bad.
On top of which it is cheaper for you , particularly if you are trying something for the first time and don't know if people will like your designs. While there are some sites listed in this thread that do stickers, keep in mind that they're raking in a lot of profit off you.
(If you're really ambitious, then you can score the paper with a good exacto. However, I think it's possible that unless you have really good hands you'll rip a few along the way, so cutting them out seems better.)
There are also sticker making machines that can sometimes be found at craft stores. These affix something on the back of whatever you run through it. I think the start up cost though is not going to be for the firsttimer.