The place where I fly from has a bulleting board, where pilots can post
their ad indicating their need for a crew on specific dates. You can
try that. Also, when I was new at this pastime and seeking to get some
experience, I placed my own ad volunteering my services as ground crew.
I crewed for many pilots, learned a lot & spent a lot of time chasing
gliders in the Western states. I also experienced many bad meals and
nights in cheap motels in the boonies. It was fun, though.

Cheers, Charles