Re: WTB H&K Mags
+1 on the above comments.
Additionally, the 10-round HK45 mags, while they will work, hang out the magwell of the USPc at least an inch or so. Just go into it with that knowledge before you buy as it does somewhat compromise the concealability of the USPc by increasing the footprint of the grip/mag area a good bit (depending on your build, method of carry, etc. of course).
As far as the best places to buy...hit the For Sale Forums at HKPro. Deals pop up there all the time for both new and used USPc and HK45 mags.