The big box stores might get fresh stock in for the fall planting season--so check back. They sometimes surprise me with what they get, although I've never seen catalpa anywhere. I see flowering pear for sale everywhere in the spring.

I wouldn't have wanted to plant in July either--except for the plants and shrubs that I got on clearance.

Did you ask at Andersons if they could get your picks for you? Does the Arboretum have a sapling sale in the fall?

Flowering pears are so common and overused down here, and are pretty weak trees to boot. I could send you some seedlings since they sprout everywhere.

If you are willing to start with baby trees, you could check the ArborDay foundation website to mail order.

Good luck!