Best 72v battery pack? And where to buy?

I am looking to rebuild my whole moped/e-bike. I am looking at 72v. Overwhelmed by the options and wondering what is the best place and type to buy. I'd like to go about 40mph (up from 30mph with my current 48v set up) and have at least 15 miles in range. I am looking at 72v 20AH or better. But where to buy and who to trust???