Is there more than one Once Upon A Child even remotely close to you? I would go to their website and try to find another location. If that location is slightly further, but not too terribly far out of your way, I'd call to ask if they have any bassinets on the floor, ready to be sold.
Additionally, are there any other second hand stores that perhaps don't get a lot of exposure in your area? We have Once Upon A Child (three of them close to me) but I recently discovered this cute, little smaller-scale second hand shop called Little Darlings that is close to me as well. They don't have as large of a selection as Once Upon A Child, but their products tend to be in better shape than Once Upon A Child, as they have to be more selective in what they buy. Either way, the prices are comparable and not all that different.
Do you have any friends that have had babies roughly 4-6+ months ago that you can borrow a bassinet from, or even purchase theirs?
I second the PP about keeping an eye out for deals for new bassinets as well.