The .357 Sig was only offered in the USP Compact line (and P2000/sk), never full-sized, and has been discontinued for quite some time.
USP-C .357 Auto (as they were labeled by HK) models were never a very high production item, either. They are VERY collectible now, and the only caliber USP I don’t have.
I did have a .357 barrel that I swapped into my .40 Compact…it worked well, but, it was actual HK. Aftermarket barrels can be hit or miss, quality-wise…I suggest if one wants to go that route to buy a surplus HK barrel. Be prepared to pay for it, though.