If you want something like a Beamspring you need to think a little backwards. Who bought them in the first place? Who might still have them? Universities, government agencies, research labs, some companies. Whoever was willing to pay 1970s prices for 1970s computing.
Once upon a time the Model M was a regular Goodwill or Salvation Army haul for $5. This is no longer true. Here is Austin we have the Goodwill Computer Works. I've found Selectric typewriters there for $20 (this is how I got mine).