@david .. older enterprise Ultra Small Desk Top PC's (usdt) dime a dozen from ebay, maybe flea markets pc's are your likely go.
I've a repurposed Optiplex 3060 i7 8700T, micro desktop,, but sata m.2 not nvme, 1 additional sata port doing a 2.5ssd.
plenty of HP Elite Mini pc's too, they call their offerings USFF. Its happily running a #Joyent (#opensolaris derivative OS)
I guess if you climb the model scale a bit, downgrade to i3, or go a bit newer, you'll find nvme, prices are growing because DDR4 SODIMM's are rarer now. so 'Cheap' is relative.
I had to spring for some new, after oem DIMM fried. Cost me $150 for 2x no name 16gb 3200 Dual rank DIMM's. Want play with zones (jails), so I paid.
Focus on the ram size to what you want first, parts vendors are up selling DRR4 for outrageous money when compared to DDR5. It's reflecting in the secondhand PC market pricing at the moment, so don't forget to memtest86 it when it arrives if second hand.