How you handle this depends upon your relationships. Your mom's best friend and her boyfriend both said they'd help, but did they say HOW they would help ? Also, what kind of relationships do they have with you ? If you are asking us what you should do, I'm thinking you don't have the kind of relationship with them that would say, "How much money were you thinking of helping with ?" It's possible that they weren't thinking money at all, but moral support, getting Mom there if it was a surprise, stuff like that. The best time to get help is when you need it, not AFTER you've done it all yourself. You probably should have asked one of them to pick up plates, napkins and stuff like that, and someone else to go purchase something else that you needed. But to do it all yourself, and THEN ask them to ante up is REALLY difficult.
Honestly, I think this is one of those times where you just have to suck it up and live and learn. Since you would have done the exact same thing had you been planning to pay for it yourself, then I think you're pretty much stuck in that boat. Next time someone offers to help, you should nail down HOW they want to help, or call them before the party, and say, "the other day you offered to help with Mom's party. How involved would you like to be ?" If I offered to help someone put a party together, and they never took me up on the offer, I would not be very happy with them if they later sent me a bill to help pay for it.
Probably not what you want to hear, but I don't think there's any graceful way to say, "Hey, were you going to help pay for this?" after the fact. Perhaps if you bump into them and it comes up in conversation, and they remember that they offered to help, they might ask if you'd like some money, but I think it's water over the dam at this point.