You may want to check threads over on the U-Haul Facebook site. [closed group... you have to join to participate] They've had many discussions of just this problem... unique bolt pattern/spacing and wheel offset. There's a parts cross-reference chart in the "Files" section of the site.
In theory at least, you can still get a rim from U-Haul. I get the impression that the degree of willingness to search one out for you varies a lot from one U-Haul shop to another.
Hubs can also be changed out, in order to give more wheel options.
I recently picked up a space-saver spare with a compatible bolt pattern as an emergency back-up unit...... from a used wheel & tire dealer. Offset wasn't the same, but the tire was narrow enough to fit in the U-Haul wheel-well anyway. [Note: most people frown on the space-saver spare option.]
Best of luck in the hunt....... Gary
__________________ "You'd care less what people think of you if you knew how seldom they did." E Roosevelt