Ryan, I'm not sure where to buy them. A lot of people grow them, so you might be able to get a sample that way. If you are over at the UT Arboretum in Oak Ridge when those are ripe, you're welcome to take a sample or two. The ones there are seedlings resulting from a study. They should be a bit larger than your average pawpaw fruit, and there's a decent chance that they will taste at least OK.

One key thing to keep in mind though, is that different varieties are going to be different (like apples). If you decide to grow one, a good named cultivar would be more likely to produce good fruit than a seedling.

Here's an outfit that sells and ships fresh pawpaw fruit, in season

Here is a link that might be useful: Integration Acres

I heard the other day that Krogers is selling them. I haven't checked it out yet myself, but that seems like a good place to start!

Rocky Point Farm in Rhode Island ships them for a reasonable cost.

Here is a link that might be useful: Rocky Point Farm