You can go to a local heavy truck salvage and pick up a used one.
You can go to a truck store and order an aftermarket one.
They have them available online as well (Amazon or eBay) Most air ride seats should be compatible. They just bolt through the floor with the nuts on the bottom.
I had my mind set on Prime seating or legacy silver leather seat once my build was done.
https://primetpi.com/
I ended up just picking up a pair of brand new takeouts locally from someone that bought a $500k RV and didn't like the colors, but I have a make a custom bracket as they are for a Freightliner M2 and adapter plates cost more than I bought the set for. Upside is that they both swivel.