Success!

I called the local Tourist Info board, and they were able to direct me to a company that rents out wheelchairs, and it's (more-or-less) in my budget to rent some for the weekend! They don't make arrangements more than a few weeks in advance, though, so I need to call them again in early July, but now I know 1) Who to call, and 2) That they do rent out wheelchairs, and that's at least half of this particular battle!

Time to go add this to my Hit By A Bus notes.