While I certainly don't know the policies of ALL non-profits, I am not surprised that the local Re-Store was unable to assist with donating supplies.  They have some very strict policies in place, not because they are heartless (they do make sure that proceeds of the sales of all items go towards the Habitat cause), but because I'm sure they would have to be fair in their policies of donating to everyone or donating to no one.  Additionally, this would be very similar to policies in place at your local Goodwill or Salvation Army that keep them from giving their merchandise away.  They make their money by taking donations, not giving them.

On the other hand, I'm sure you could find out where the Re-Store gets their donated materials from and request assistance from them directly.  Our local store had lots from Lowe's and some local construction companies.  If you were to ask them for any doors that they didn't need, they might be happy to help out!