Different letters pose different challenges. "K" for example, is going to be really hard to find, while the problem with "O" is what particular O of the thousands of roundabouts, pergolas, pagodas, irrigated fields, silos, etc, to choose.
I thought j would be difficult, but I found one amidst the sands of Iraq.
There are loads of small (but perfectly formed "U"'s in the vicinity too, but I want all the letters to look different if possible, and U should be fairly commonplace.
The "j" is here by the way...
30°23'48.63"N, 48° 4'43.02"E