hardware options Netgate sells their own appliances( they write/maintain pfSense) the devices are a little pricey but also installed and operational, guaranteed to work and newer faster updates. You're also supporting the maintenance of a free enterprise level router software that runs mega corporations. 

Used are plentiful and cheaper Most any single processor 1U short depth server like Dell R210ii, R230 or similar relabeled, Datto 1U, Hyve Zeuss 1U, 1U Supermicro etc Many already have Dual 1Gb NIC but a 10Gb card can be added 

There’s also Mini PC routers like Protectli, Firewalla, Qotom Q555G6-S05, Intel NUC, Beelink EQ12 and many more

Thin Clients with an expansion port like HP T620 Plus or HP T730 and add a dual port Intel Nic up to 10Gb/fiber etc

Mini PC ThinkCentre M720q, need a p/n 01AJ9040 pcie riser card, i350-T4 v2 4 port RJ45, x550-T2 for 2 10Gb RJ45, i226 4-2.5Gb RJ45 More info: https://forums.servethehome.com/index.php?threads/lenovo-thinkcentre-thinkstation-tiny-project-tinyminimicro-reference-thread.34925/

Smaller pc’s like the Dell Optiplex are also good options with extra slots to add NIC like upgrade to 10Gb/Fiber etc