I have been collecting Harn stuff for a while now. The best site I have found is:
http://www.lythia.com/index.php
There is a complete product list on that site somewhere. There is also a forum called harnforum linked to that site, which seems to be the best forum on the net for Harnpeople.
I bid on Harn stuff when I can. The prices really vary. I have got some materials quite cheaply, but because there is not too much of the early material listed, the prices sometimes shoot up.