I don't think there's a standard "goto" place for that sort of thing, generally I only see those type of roles advertised directly on the startup's website or v

I don't think there's a standard "goto" place for that sort of thing, generally I only see those type of roles advertised directly on the startup's website or via their twitter feed. You could probably do something clever by taking some twitter lists of startups and searching them for keywords like "admin".