Any Take Out B'stilla in Los Angeles?
B'still I think, perhaps because of the egg layer, is one of those things that really needs to be fresh. Maybe I am being overly purist here, but the only solution is to take the day off, get to the party early, and make it yourself. And, actually, you can make the squab (or chicken) tagine on which it is based a day ahead, and you can prepare the almond paste earlier as well, so you only have to show up an hour or so in advance.
It is worth the effort, and if you bring it out fresh and puffed they will know you are a fanstastic cook.