You can buy one from HK and install it yourself. The part # for a full size USP 45's hammer strut is 214819.
Contact them at cs(at)hk-usa(dot)com to see if they have the part. Tell them that the HK-USA site appears to be down, and ask if they can send you an order form through e-mail. Fill out that form, turn it around and send it back to 'em. And if nothing goes wrong, you should have your part in 7-10 business days.
Then do a search on this forum for the detail strip of a USP. You'll have to do some mid-level difficulty surgery to your USP to replace it.
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