I don't know the price, but I would assume any H&K dealer would get them through Nichols in Montreal. If you're in Ontario, Simmons has been getting H&K parts quickly as long as they're in stock in Montreal. If not, the boat from Germany takes 6 - 8 months.
There are several barrel makers in the U.S. that have drop ins for the USP, but your aware of the import issues.