You can get DEF at basically any place you can get motor oil. Parts stores, C stores, Wally world, heck saw some at Home Depot.
Not surprised there aren't many 3500 gassers. Just not a popular combo unless you're a city maintenance department or ma Bell!
As far as reliability, I had a fleet of 2014 Cummins pickups, new.
Same as '15s except the '15s have a few upgrades.
These trucks were operated all winter on the N Slope of AK in what is probably the worst torture test for a Tier IV diesel emissions system.
WAY Below zero for months, never over 35mph, never put under load, some idled all day, some all day and all night.
After 7 months of work, Sept thru April, the trucks had anywhere from 1000 to 4000 hours on each of them. I think mine was the least with just over 1000 hours and about 1500 miles! lol
Aside from frozen DEF injectors needing replaced, freezing the def filler tube solid when trying to refill outdoors, many manual forced regens, I don't recall any hard parts needing replaced except def injectors. One Ram 5500 got so plugged up that after multiple regen attempts, the ECM locked out the regens and we forced it to think the dpf had been replaced and it cleared up after a few more regens.
Bottom line, it requires a little more diligence, but I believe the systems to be largely reliable.