Hello!
This is a controversial topic. Apparently H&K has provided a statement of some sort (someone who knows this better feel free to correct me) to certain law enforcement agencies stating that they never imported Hi-Cap magazines for the HK USP45f prior to the ban.
Now these magazines DO exist, however, and some say that those that were purchased by a private party in europe prior to the ban are technically legal for transfer.
Others maintain that the VAST majority of the "pre-ban" magazines availible for the USP45f in the US right now are just non-stamped post ban magazines brought into the US greymarket.
Whatever the case may be, they are DAMNED expensive, usually going for $100-$150 each, if you can find them at all.
If you decide to get them, I would suggest that you request a statement from the person who sold them to you stating something to the effect that the magazines that you purchased from them are "pre-ban" as defined by that particular crime bill.
Good luck!
PS I know a "friend" who read up all he could find about this and decided to purchase them anyways with a statement. He managed to find them for $100+ shipping from a private party seller who contacted him through an e-mail address and did not respond to wanted requests on bulletin boards. Some bulletin boards wont even let you ask for these or sell these at all.
as usual, YMMV, FWIW, IMHO and such!
J.T.