There are far fewer HPNA cards on the market than there are Ethernet cards and they much more expensive than Ethernet cards.

http://www.linksys.com/products/product.asp?grid=34&scid=33&prid=130
http://www.langalaxy.com/showroom/1163.cfm

Though of course if you don't have an Ethernet network already then you will also have to buy cable and a hub.

Do you really need HPNA, in many cases it is just as easy to run an Ethernet network instead.