devtrackers.gg
You should be able to access it here: https://rs.runescape.com/necromancy#NecromancyPackages if you click on the "Members Founder Pack" link and sign in with the account you want to pick it up on, you can do so there.
I haven't seen the Founder Pack in the Marketplace myself, but I already have it so that might be why for me, specifically.