How cheap? If you can’t get on the air for $0 you’re not trying hard enough.
Where do you find these generous guys? You attend club meetings and ask around. Personal contact is essential when you’re looking to borrow or buy a rig at rockbottom prices.
Older tube rigs like the Heathkit SB-series or Yaseu 101s or old Drake rigs make great first rigs. For around $200 to $300 you can get into some older transistor equipment (There was a Yaseu FT-747 for sale on the Ontario Swap Shop for $295 which would be a great first radio.)
BTW when it comes to online swap shops often the prices are inflated. Better deals can be had at regional Ham Radio flea markets.
Dealing with someone you know is the best way to get a rig that works for very little cash.