I'm finding that I am needing to connect two standard 3/8-16 (coarse) bolts at a 90 degree angle for mounting, like an elbow fitting would do. I need to be able to thread them in individually, because it's a tight space. But I cannot find any sort of nut or hardware that can thread in to it. I thought "3/8" street elbows would work, but they are too large. Suggestions?
Do think you will have to make up something.
Street elbows are for plumbing and thread size/type does not match.
I would use a long type of nut(as shown in @JPH1618's answer), and cut it in half at 45 angle. Weld the two half's together to make a 90 degree angle.
Could also weld plain nuts to an angle bracket.
There is such a thing as a hinged threaded standoff, that is basically a non-rigid version of what you're looking for. (No endorsement implied, in fact the linked company doesn't even carry them in the size you need.) They are available with every combination of male & female threaded ends.
A block of aluminium for each side. The vertical hole may need tapping, but the horizontal one could be drilled straight through, as the bolt going that way would need to thread into the drill (judging by the diagram).
It may even be possible to drill straight through both, depending how the vertical thread needs to be fixed.