I’d say mostly eBay, I believe often times since fpo’s don’t sell very often, listings get taken down and put back up after some time.
Also your location matters as sometimes listings exist on other various eBay sites that aren’t visible to others ( .ca, .com, .co.uk, etc.)
DFW pokemon was the team that opened a ton of packs last year
I guarantee anyone interested could contact them and get a set or a specific card for a reasonable price but I have never requested so I’m unsure.
Regardless, fpo’s are still rare and unique! Awesome to see more listed! If only they were auctions 