Since the iPad is not available directly from Sprint (at this time) you have two choices. You can purchase a phone from Sprint that has tethering capabilities or purchase a mobile hotspot from Sprint. Each will provide 3G access (and in some cases access to Sprint's 4G network) for your wifi only iPad.
Obviously, if you already have Sprint phone with tethering available you need not purchase another phone and your iPad and phone can share a data plan. Unless you're prepared to break your contract with Sprint and possibly to "root" your Android phone, however, you will have to pay a monthly "tethering" fee to Sprint. That fee may or may not include additional download access. You'll have to check with Sprint.