Apple doesn't not have a give-a-way program. It does sell refurbished Macs that are guaranteed like new Macs.
If you purchase a new iMac and have an Apple Card the interest rate is zero%.
The only other source for a refurbished iMac that I would use is OWC (MacSales.com). They are considered the premier 3rd party hardware supplier for Macs by most of the experienced contributors to these forums. Do not buy a refurbished or used computer from any other source as you can get stuck with a very expensive door stop.
Another option would be to check so see if there's a Macintosh Users Group (MUG) near you. If there is contact them to see if they know of any used iMac for sale. They would know how to prepare it for resale properly. The MUG members may know of free Macs that may be available thru their members.
Also be aware that if you get a newer Mac with a newer system you may have to replace all 3rd party software. With the introduction of Catalina (macOS 10) any 32 bit app will not work as Catalina and later require 64 bit apps.
