Any kind of sticker will work; there's no need to spend a lot of money on stickers. You know those free return address labels you might have pages and pages of? Just cut your name and address off and use those!
Stickers are also a fun way to end a lesson. No matter how the lesson went, it's a positive note (ha ha) to finish on and something the student can regularly look forward to. You can also use stickers throughout the lesson (especially of you're swimming in stickers like I might be :) ) when the student does something particularly well. The student can place them in his/her book, on or in his/her lesson journal, have a sticker page, or even just wear them on his/her shirt.
The students might or might not act like they like stickers, but chances are, inside, they'll like them.
Do you use stickers with your students? How?