There are many ways to find one locally or on the Internet.
In North America, there are many classified sites like Craigslist where often you will get un-serviced tape recorders for lowest prices and then there are more audio focused sites like US audio Mart or Canuck audio Mart.
A very good all-purpose site for Searching audio https://www.hifishark.com/
You can also find a great selection of reel to reel tape recorders on this site
Then of course there’s eBay, which is where you will usually pay top dollar.