First, put your place or residence into your profile, so someone here might steer you to a resource.
Without knowing where you live or any specifics about the crank I can't give you definitive odds, but it's very likely that you can find a comparable (same length, fits spindle) crank at a bike co-op. Many co-ops get large numbers of donated bikes of the same era as yours and strip them for parts, or cannibalize them so they can make one usable bike out of two or three relics.
So a co-op is the place to start, and if I knew where you lived I might be able to steer you to one.
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