A 501 c(3) non profit must have available, for viewing and copying, their IRS 990 return. Anyone can walk off the street and request this form. If it is not available, it should be reported to the IRS. The IRS would then have cause to fine/investigate. Wherever this office may be, someone should take a walk and ask for their 990. It'll tell where the money comes from, assuming that it is completed accurately (and I don't hold much faith that the 990 will reflect the "real" set of books of the organization...).
I already sent the URL for the pdf of their 1998 form 990 which was available online.
It doesn't show who the top donors are, but it does have the officers and there are some mistakes that they made in filing.
FReepmail me if you want to take a look at the URL.