E.C.
I am sure you have gotten lots of advice already, but I will add my two cents anyway :).
A couple of years ago. I had a ball themed party for my then one year old as well. I wasn't happy wit any of the ball pits that I found for sale, so I "made" one by filling a wading pool with ball pit type balls. It worked great and the kids loved it! I think I bought the balls at Toys r us. Good luck, and have fun!
-E.