If you want grafted trees, those are available from a number of nurseries you can find on the internet. Many of those sell seedlings too. I have bought seedlings from the Missouri MDC and have another 25 on order for next year. A few other state nurseries offer them too but MDC is cheap and easy to buy from.
Seeds are bit harder to find from a good source. You don't want seeds that have been allowed to dry out. I'd buy stratified seed ready to germinate from this place- http://www.nuttrees.net/pawpaw_seeds.html. They sell grafted trees too.