David is correct - HKS does make a mag loader that will work with most all HK's (except the PSP and M8 mags - too thin). HK also makes or made a loader at one point in time. Looks like a black block with the HK logo. Not sure how good it is - never used one before. There is a shop in Dallas that has/had them.
H&K makes a loader with removable innards to adapt to different mag shapes. A while back I bought one for a P7 from a firm in Maine that the name escapes me and they have a web site. They produce AR-15 clones and handle H&K parts for rifles and pistols. The mag loader seemed to work ok for my purposes.