I was at a thrift sale and I found a super pink prom dress that started the whole Homemade Adventure Time Family Costume. Not perfect, but it worked. Found a white hat for Finn and made his backpack out of a Goodwill t-shirt. Cut a hole in the hat and used the fabric to make the ears. Finn couldn't wear shorts and a t-shirt as it was 50 degrees out, so we reverted to a blue turtleneck with the blue shirt over it and some navy pants for him. My husband bought FOUR extra large yellow shirts, some felt and a sword from the craft store and made his entire Jake costume out of only three of the shirts, in a couple of hours, including the "hat" with the Jake face on it. Bought the pink wig and made my own crown for Princess Bubblegum.
We had a great time creating the costumes, and a great time at the party.