There is nothing wrong with asking for money at this type of an event, there will be a fair amount of people, and you are picking up a 'specialty' item that we normally don't have.
Justin can amend the first post, but you have to be certain about how much you are willing to get as far as food/any utensils(skewers), so that you can be reimbursed.
If you are that concerned about asking for money, what we typically do is ask anyone who has purchased something for the event to keep their receipt(s), and they are reimbursed that way.