Why wouldn't a florist donate? If you let them provide a floral decorator to teach the class (vs. the woman you know), and if you took photos and/or invited the local newspaper, the florist would get free publicity providing a service to kids. I realize you don't think kids are good customers of florists, but they are future customers, and their grateful parents would think kindly of the florist! Does the organization have a newsletter or e-blast list? Send photos of the class out to everyone on that list.
I'd ask the local craft store to donate some vases under the same conditions. Why not? It costs them next to nothing, and they get good will.
I've done an unbelievable number of fundraisers, and you'd be amazed at what people will donate to get the word out.
Assuming the organization is a non-profit, community group, I wouldn't think you'd have much trouble. You can stop in to some florists to ask for "advice" and see how receptive they are. Polish up your speech first, and see how far you get.
You could also call the local Garden Club and see who they use. Anything that builds future flower lovers and gardeners is a good cause.
Anything you get from a funeral home won't be something they can plan, and they may have a system already where leftovers are donated to nursing homes and so on - but you won't be able to plan on anything. If nobody dies or the family wants the flowers, you're stuck with no program!