With orders of $25 or more, Amazon offers Prime members this Wilson Championship Regular Duty Tennis Ball Can for $1.97 with free shipping. (Non-members can spend $49 to bag free shipping.) That's about a buck under what you'd expect to pay at a local sporting goods store. They're designed for clay and indoor courts.
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.