When I don't want to buy books I hunt the internet, there's a web-site that has tons of printable math games (you can click on the grade level). Some might need some prep, but I always like to have at least one or two games in my centers.

http://www.dr-mikes-math-games-for-k...ath-games.html
This web-site is full of printables, some might just be if you finish early type stuff, but it's free:

http://printables.familyeducation.co...=skill-builder
Good luck with your centers, I haven't taught math in a while but my I helped my co-teacher set up her centers and I use some of these when I tutor in the summer.