Having organised fund raising before, it is always worth writing to local large companies and asking for any donations towards your cause. Local garden centres might be persuaded to donate some gardening type items, probably worth making an appointment with the local managers and seeing in person. It's a bit trickier as you are not a charity, and most companies prefer to donate to them (tax reasons!), but big companies may still offer something; ...don't ask, don't get! ;D
The other option is to ask your plot holders for donations of unwanted goods, even if it's a couple of packet of seeds each, you could put together a few prizes.
Good luck! ;D
