As an alternative, can anyone recommend any online plant vendors I might check with?

I think what you consider root bound is TOO root bound. From that photo it looks to be in a no more than 6" pot,maybe just 4". It should be at least in a gallon pot. A lot of plants will put up with less light and being under potted when young...but if you don't upgrade conditions,then a decline will set in as it ages.

When we got rid of our T.V. armoire, we ran into the same problem when hanging the T.V.. Did not want to purchase new furniture just for the T.V. so we decided to use an existing furniture piece. We balanced out the look with side chairs flanking the furniture piece.

My first guess would have been spider mites, but I don't see any signs of webbing. There is a slight possibility that too MUCH water is the problem. Sometimes when abundant water and high temps force new growth, you end up with anemic, chlorotic leaves (though yours would be about the most extreme case I've seen. And I'll spare you the soil science, lol). Hit the affected roses with a dose of chelated iron. If that's the problem, the symptoms will improve almost over night. In any case, the iron won't harm the roses as long as you follow the recommended application rates. It can be applied both at the roots and as a foliar spray. Since your temps are so high, I'd go with soil application, unless you do it late evening. Wet leaves overnight shouldn't be a problem, since black spot goes dormant when temps are over 90F.