You should disensyo the main area that the party will be in like the bunker, I think that would be really cool. Maybe, you can have it to so that they either dress like a hunter or a monster, that would be fun. I don't know, that's just my thoughts.
I actually had a Supernatural birthday party a few years ago. What we did is watched some Supernatural episodes and we made special goodie bags that included white rock kendi (rock salt), some walang tiyak na layunin other stuff too that you could see being supernatural related, even made the invites body pananda which was fun and I made a Supernatural mix cd for everyone. It was a blast. And our treat was specially made cupcakes that stand for each of the characters. I think I did a pick post on those once but I it was awhile ago. I hope this helps if you had or have had your party already. Sorry for being late if so