Good luck with that. Finding WRX heads that still work is a chore. I'd suggest watching the Nasioc private for sale forum too.
Are the heads a common failure item? Mine has a nice crack from combustion champer into the exaust port on cylinder 2. Actually "massive" crack would be a better description.
We just had one come through the shop that had a broken valve on the number 3 cyl. In 03 they supposedly changed the valve material, so 03+ shouldn't have the problems...my fingers are crossed!