UHMW would be perfect for chain tensioners because has such a low coefficient of friction, and is so wear resistant. If you could make some spacers with an ID the same as the bearing block holes, they could fit over screws and slide freely on axles with minimal slop. It could also be very cheap at $10/8ft from online metals: .5" UHMW rod. It could be sold in long lengths, and then cut to size with a hacksaw, or in ~.5", 1", 1.5", and 2" lengths (pre-cut, with loose tolerances). These longer lengths would be nicer than the .5" lengths of nylon spacers, which also move around a lot on the screw/axle.
If it’s that easy, why not do it yourself? Only I think teflon would work slightly better as the coefficient of friction is lower and the amounts needed are small anyway.