Hi Frabon,
Welcome to Laptop GPS World.
Which USB GPS unit are you looking for? Is it one of the older models, or is it the one that came with the new 2009 version of Streets & Trips?
The older one used to be possible to buy directly from Pharos, the maker of the unit. The newer one is made by a company named Navation.
If you have a difficult time locating a store to order it from, you could buy a different compatible type, such as the popular BU-353 by GlobalSat.
But even though the BU-353 is so inexpensive, it is possible to find Streets and Trips w/GPS for only a couple of dollars more!