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Short answer is yes, if you search for M5 Standoff Spacer on eBay etc you can get various stand-off lengths with an M5 bore. The tricky bit can be finding the correct outer diameter.
Top hat sections can be harder to find. I usually have to get these made and it’s expensive.
You may be able to 3D print something if it’s just locating the screws and not load bearing. Also search for 8mm brass or aluminium tube with 5mm bore and cut to length.
Is there such thing as a sleeve washer if I wanted to put a m5 bolt through an m8 hole and take up the slack.
Why? My motorcycle has after market bar ends with m5 and the expander bolts are stuck. I want to fit bar end mirrors and rather than fucking around with removing them I'm just going to reuse them. However, the mirrors have m8* bolts and holes and I want to keep everything as snug as possible without over torquing given I'm using an underspec'd bolt. The hole has a chamfer and the head of an m5 won't fall down...but still.
*I actually need to double check this