Just go to an auction and 'rescue' your own. You can buy some pretty nice yearlings and 2 year olds here at the local auction for $100.00 to $1000.00 depending on breeding and looks. Some really well-bred and really nice one go through for under $500.00 every sale.
The main thing with any new horse (but particularly from an auction) is to isolate them for 2 weeks. We give young ones Penicillin for 3 days. It will keep them from getting Strangles if they are not already carrying it. It won't help for the Flu or Rhino.
If you handle a new horse, do it at the end of the day and do not go and handle your other horses after handling them. People and dogs spread most diseases from new horses to other horses.