Have you considered Kickstarter?
The only reason I mention it is that I was thinking the tricky part is figuring out the actual number of units needed...
If you think you can get a great price break at 500+ units, you could start a kickstarter with a project goal to be able to pay for the 500 units.
At $xxx dollar donation, the person gets a unit when it's done.
If it works out, great...
If not, no one is charged, but you have a more realistic number to go by...
You can have tiers as well to cover resellers. So it you kickstart $xxxx you get more units (bulk purchase)...
Probably a bad idea, but I've been engrossed with Kickstarter recently.. ;-)
Anyway, good luck with this..
My 500 and I will be watching this closely..