, vintage machines are not anywhere near as prevalent as modern machines, so they're harder to find. And finding one in working order is even more difficult.
Hmmm.. I think this varies a lot by where you live. I'm in a big city and if I put "sewing machine" into my Craig's List I get 50 or 60 listings, mostly older machines, better than half I'd be willing to test out & set up a beginning sewist with... The prices vary wildly (all those people who think their rust seized treadle is worth $1k :falls over laughing: ) but in general it's a matter of knowing where to look.
Shopping for vintage goods isn't like shopping for new things, if it's listed on-line somewhere, the person listing it either knows it's value is worth the cost of shipping, or (more commonly) they have an over inflated sense of it's value. It's when people don't think it's worth anything
that they donate it to charity or list it on Craig's List for $50 in order to get someone else to remove that 40 pound dust collector. The Salvation Army nearest my home has at least two sewing machines for sale every time I go in, usually running & under $50.
Another advantage to thrift store machines is they will often let you test them in store, provided you bring your own fabric & thread.