>>1624845You can either buy a vintage road bike with 120mm rear spacing and apply a fixed gear conversion.
Or you can buy a modern bike and put a 6 bolt fixed cog in place of the disc brake rotor and flip the wheel around. YMMW with the chainline though.
I did the latter as my first fixed gear just to try it out, ended up liking riding fixed gear and bought a legit dedicated fixed gear bike.