IDK if my idea of a bike co-op and yours is different, particularly in the discussion above about "quality parts".
The most successful and longest lasting co-op I am aware of here near Atlanta has tables and tables of what most enthusiasts would consider junk or BSO. Very occasionally you can locate a nice older machine and in particular is a good place to find bikes/parts for something you may wish to commute on and not worry so much about theft.
Given their stock I would figure that a good start would be a municipal police impound. Find a couple of people willing to sit there and hopefully with some basic level of maint. and assy. ability. Beyond that, some money to keep the rent paid and lights on, according to your access to a "free" place to be (don't forget zoning if such should apply).
The co-op here used to be pretty much free but did ask for donations. I think they swapped over to a payment system some years back in an effort to keep money coming in and stay open. At one point they were also willing to allow barter and in particular trade for labor.