I'm not sure about the iMac Pro, but he 27" iMac is super easy. Just pop open the little panel on the back and swap out the RAM for the new RAM at 1/4 what Apple charges. This is something you can do by yourself.
I would probably buy from OWC https://macsales.com as they have nice configurator to get you just the right RAM you need. For example you might have four slots with 2 16s already in 2 slots, so you could just add 16. But you might have 4 8s in 4 slots, so you'd have to lose all of the 8s to put in 4 16s.
But OWC should have the info on your model, and if not it should be pretty easy to figure out through the system info