My standard advice is to look for late 70's and 80's Japanese bikes as a starter names like Miyata, Centurion, Panasonic, Bridgestone, Fuji, Nishiki, Univega from there look for stickers that say cro-mo or double butted or brande names like listed above.
check with co-op rules, I know some charities that don't want volunteers grabbing the good stuff without ok
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Life is too short not to ride the best bike you have, as much as you can.