Some taprooted trees are difficult to transplant so you rarely find them in local nurseries.
There are some websites that have pawpaw trees for sale but I don't think it's common to grow them in zone 4. Your native pawpaws might be more adapted to your winters than the named varieties. You may have trouble getting the named ones to survive if you did order some. Or is a named variety what you have now?

Well, I think that it is changing. A LOT more companies are offering them for sale. When I bought my first one almost 20 years ago, the only mail order source I could find was Raintree (mind you, this was before I knew about the Internet).

Now, there must be several dozen mail order companies that offer them, from specialty to general mail order nurseries.

And yes, I too find them extremely ornamental. I suspect, though, that the fruit is the main reason they are NOT grown as an ornamental -- many people don't want anything that is "messy" and drops large, squishy fruit.

Finally, paw paws do have a bit of an aftertaste, sort of a medicinal flavor that takes some getting used to. I have heard many people comment that they are a little "weird tasting" - I know that I was SO anxious to taste my very first ripe one, and I must admit it did have a bit of a Listerine aftertaste to it. Now, though, I have learned that 1) most of the aftertaste is in the skin and the layer of green cells right under the skin, and that can all be avoided, and 2) like many other delicious foods, such as olives, artichokes, various ripened cheeses, caviar, etc., paw paws are an acquired taste, and the more you sample them and use them in various ways, the more you will like them.

Some of the companies that offer paw paws include Raintree Nursery, One Green World, Burnt Ridge Nursery, Hidden Creek Nursery, Stark Brothers Nursery, Indiana Berry Company, Nolin River Nut Nursery.

I have a couple of seedling paw paws from Burnt Ridge that took a few years to get over 6' and start to flower, but are 10' or more now. Only one sets fruit, the other drops all the tiny fruit, like one can pollinate the other but not vice versa. I bought a little grafted tree 3 years ago to try to remedy this situation but it is extremely slow growing, only getting a few inches taller each year. So I'm starting to wonder about reduced vigor in grafted trees, which everyone always promotes as somehow better and therefore more expensive. I bought a couple of grafted persimmons, and was disappointed to lose both of them in the first year, they barely broke buds and failed to grow. But the rootstocks are still alive and trying to grow, I just don't know if they will ever have any good fruit, but I will wait to find out. I had also bought some seedling American persimmons back when I bought the paw paws and a couple are now about 6' tall, I don't know what height they will have to attain before they will bloom, nor what sex they are so may not be able to expect fruit even then.

I do like the paw paws, and I have peeled them so have not noticed any aftertaste, just a sweet banana-like taste and consistency.

One thing is that persimmon leaves can be used for tea, not a total loss.

I have two in the backyard. They dropped all leaves their first 2 summers, despite having 1-2 hours of shade each afternoon in July-August courtesy of a patio umbrella I positioned between them.

I think the direct heat made them stronger as they don't suffer from weather-related problems now.

The fruit drop is a problem - its 100 degrees here when they fall (and they aren't really edible until they drop) so you better scoop them up within a few hours or they'll go bad. The seeds are large and if you let the fruit rot you are left with quarter-sized fruit (and the aroma of decaying fruit - not horrible as the pawpaw smells good).

No one in my family enjoys the fruit but the tree is still awesome to have in the yard.

Propagation tip: arm yourself with a handful of q-tips. Go out and rub a q-tip inside the blossom on one tree. Pollen will attach to it - rub that inside the blossoms of the other tree. Do this back and forth between trees every 3-4 days and you'll wind up propagating a decent percentage of fruit.

This is the third year my 12 or so year old tree has bloomed, but it has never borne fruit. This year, the tree was loaded with flowers, and now there are many rice grain sized immature fruit. Can I start counting my pawpaws before they're developed, or can I expect most, if not all, to abort?
Pics in the link, if it works.

Here is a link that might be useful: Pawpaw 21 April 2012