Every city has a auto dealer who handles trucks like that. UPS has their own custom trucks so you can't get that model, but there are a lot of options.

You won't find them in the typical dealer though. They are a niche, and most customers are buying several at once for their business and so the way the whole process works is different. Though there are just enough small buisness buying exactly one that you won't confuse them by buying one - but they may have trouble figuring out what tax applies.

Same place UPS does: https://morganolson.com/products/parcel/

They're called step vans and available anywhere commercial trucks are sold: https://www.comvoy.com/bodytypes/Step-Van

You can find used ones in running condition relatively cheap.

It’s called a Ford Transit.

Mercedes makes a couple of models as well if the idea of driving a Ford offends you.