Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US - Hardware Questions and Recommendations - OpenWrt Forum
Buy PC Engines ALIX APU Online. pfSense OpenWRT Routers.
MOTORI PC APU 1D4 - 16 GB SSD / 3 LAN / router / firewall / OPNsense / pfSense EUR 160,00 - PicClick IT