For practice balls I would see if you can find a TD who provides balls for their tournaments. We used to buy their slightly used game balls for $1 each at the end of the tournament.
For hitting we have every ball imaginal, for pitching practice we try to get a game used ball. Like JAD said might be able to get balls cheap. I am not even trying and have about a 1000 of the dang things.
Actual make of ball changes so we have a few sets for pitching.