I may have mentioned it in another thread, but I think in this case a press release would do wonders for getting sponsors on board. Tugging at enough heartstrings would surely get sponsors wanting to be seen as the benevolent ones, but depending on the sponsors, I wonder if they’re taking their time only because they don’t see any immediate benefit for themselves. (N.B. I realise how cynical that sounds…I swear I don’t hate humanity that much).
Also, the emails you’ve sent may have gone into a large pile of emails in their “info@” box that gets checked by an unpaid intern twice a week.
If you have a few sponsors on board already, maybe you could mention them in the press release (with their permission, of course)? That would almost certainly get other companies noticing.