What kind of customizing? Wording or pictures? I have gotten mine from Vistaprint.com. Actually, I have gotten a lot of stuff from there and love it.
You might also try Walmart's photo dept. I believe they do them as well. I know you can order them at CVS and other drug stores but I'm sure Walmart is cheaper.
Received my customized mouse-pad from Vistaprint,it looks great and the shipping was super fast,very pleased!!