Personally I would recommend just getting a newly-morphed baby. They are far more readily available, and in my opinion it is easier to care for a baby than a tadpole. I would also recommend just starting with one. These frogs eat a lot, so much that many people choose to have feeder colonies to keep up with the prodigious appetites of a single frog(often to feed other types of frogs too, but the bullfrog really makes it necessary.). I myself have a roach colony to feed my frog. His last feeding(for reference he is a 5 inch male.) ate 40 adult crickets, a small pinkie mouse, and 2 nightcrawler worms. In addition they can't share a tank, except possibly mating pairs, and many people prefer to keep them apart outside of mating attempts. These frogs can fight and kill eachother. If one baby outgrows another it might try to eat the smaller one. They're very voracious frogs. Babies are easily found at Reptile shows and often exotic pet/reptile stores. They usually cost 35-50$ in my experience. These frogs are also quite the commitment, since they can live as long as 20 to 40 years. Whatever you end up choosing to get, good luck!
Also, I'm curious, what are you planning to do with the frogs/tadpoles you have now? Someone may come on later who can positively ID them.
Edit: I meant to include baby African Bullfrogs can often be found online on sites like LLLReptiles. They are usually listed as "CB African Bullfrog(or Pixie Frog, Giant African Bullfrogs, etc.)"