I just grab them off ebay. SAS drives are almost exclusively used in enterprise servers, so there’s fuck all demand for them on the used market. Buy a cheap PCIe SAS controller card and you’re good to go.
I use Unraid which doesn’t care at all about what types of drives you use, so you can mix and match into one huge array. I have 8x SAS and 2x SATA drives in one array.