I recommend that you hire a competent Realtor to assist you with this. As my colleagues have shared most co-ops want 20% down, so do most banks, also co-op boards have the responsibility of makeing sure financially capable people buy into the co-op. Forclosures bring down the financial viablilty of the co-op and all the shareholders stand to lose.
Having said this, there are some grant programs around that will assist with down payment money and or closing costs. They also provide financial counseling for purchasers to help them plan for the future and avoid losing their home to forclosure. Hope this helps.