I buy on eBay. Just do a search for "brass sheet" or "brass flat". I'm assuming you'd want 1/32" (.031") or 1/16" (.063") of 360 alloy. You also could get it from Jantz or Texas Knife I'm sure. I would epoxy it to my knife first, ream the pin holes again after it's fully cured and then epoxy the scales. Just my method, though.
Erik