One might be tempted to ask where Batman acquires all his fantastic gadgetry. Yes, one might. For the sake of your enjoyment of Batman: Arkham Asylum, we suggest you try not to worry about it. Sure, Bruce Wayne is probably blowing millions on R&D alone (not to mention production!). That's money that could be spent funding local police forces around the country – effectively stopping crime from coast to coast – rather than spent trying to stop a handful of crazies in a single city.
Seriously, don't think about it too much. You'll just get depressed.