I called H&K not to long ago asking this same question. They offer some parts on their website webstore, but the selection is bad, really bad. www.hkparts.net seems to be the best source. However, you can find parts at Midway and Gun Parts (an outfit in NY).
The customer service rep also said that if I could not find it at the places he suggested to give him a call back and they will see what they could do for me. He should have just offered to help me there and then. I ended up getting one part from hkparts and the other part from Midway.