Look for an ex fire that you can reimage with it. I have a smooth wall S4

I’m running it on one of these.

Please elaborate. What do you expect your box to do? And how many ports do you need?

Assuming you’re not planning for anything computationally intensive such as VPN or deep packet inspection, you can take a look at the four-port Sophos SG 105 / XG 105 (will require one change in BIOS settings for pfSense to install and run) or a five-port Barracuda F12 (doesn’t have video output and therefore will require a console cable for installation). Occasionally, the six-port Check Point T-110 pops up at low prices. Very occasionally, someone lists for cheap a Lanner unit with anywhere between two and six ports. Four-port Sophos UTM 110/120 units still pop up occasionally, but those are REALLY old (went out of support in 2018).

You may happen upon a three-port APU or a rebrand thereof (SimpleWAN or even Netgate; they used to sell pfSense Plus preinstalled on APUs), but those are extremely configurable and sometimes come with stock OS installed on a CF card, so you may need to buy an mPCIe SSD separately.

And, of course, oodles of dual-RJ-45 mini-PCs, new and used. Potential problem with those is, the cheaper they are, the more likely they are to come with Realtek NICs (and those NICs used to have a big stigma in the pfSense community due to poor drivers; more recently, things have improved, but many people still don’t want to deal with Realtek). Also, if the seller doesn’t know or doesn’t say what the NICs are, they are probably Realtek.

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