I bought a rear mudguard as I'm not happy with a cold wet bum. I bought a rather good X Blade II from Tweeks Cycles. It's a good fit, adjustable in a couple of places. It fits to the top of the frame tube where the seat post enters. It seems to have stayed put so far. The bike is a Levo Full...
Try eBay. A seller called Kayfast is one of my favourites and they will sell small quantities. They do lots of Stainless bolts. A2 and A4 grade. Fast delivery too.
I think the theory behind the USD forks is that they give lower unsprung weight. So the heavier part of the fork is mounted on the bike and the wheel only has the lighter fork tube. The aim is to improve the handling by reducing the unsprung weight.