seeing that I would be very concerned about the frame.
by the way, the front & rear frame sections join together at that mount and rust really likes to hide between the 2 plates.
I on the other hand would not be too concerned about the frame from what I can see of it. I would however take a very good look at the other three spring hangers, better to do all of them at once if necessary. Just from the initial statement by the mechanic that he can't find the parts, I'd be looking for another opinion from a unrelated source.
The mount is rusted and cracked, but that frame appears to be mostly surface rust which almost all frames will have. It's fairly common for the mounts to rust out and/or break without negatively affecting the frame. I've seen (in pictures online) the mounts rusted away to the point that the spring is resting on the bottom of the bed, but the frame is just fine to install a new bracket. Mounts are readily available as noted above.
I wouldn't hesitate to replace that mount myself, but I've got access to the tools and place to do it. Depending on the price quoted for the work, it may be cheaper for you to buy the tools too, plus you'll have the for future projects.
Jack and jack stands to raise and support the truck. Tool set to remove the leaf spring and reinstall the new bracket with bolts. Angle grinder, hammer and chisel/punch for removing the rivets. Wire wheel and paint the frame if desired. Grade 8 bolts, washers, lock washers, and nuts to reinstall the new bracket.
BTW... Two piece frame started in '98, OPs truck is a '97.