You might be able to buy tickets to a Fremantle supporters section inside Adelaide Oval via the club web-site, at some uncertain future date, by clicking on the link:
membership.fremantlefc.com.au/away-game-ticketing.
But you'll be disappointed with the answer, because, at present, they're only offering tickets for the round 4 game at the MCG. I know they're probably still unpacking boxes after the big move to Cockburn, but I'm sure they could a lot better if they wanted to.