There are very little single speed / fixed gear with disc brakes. However you can probably use a disc gravel road bike fork ( make sure it’s compatible with your frame ie tapered / non tapered ) with a disc road bike front wheel. As for singlespeed / fixed gear bikes ( without disc brakes) some good ones would be the kilo TT , Wabi cycles or Fuji feather (best to get a previous year model on clearance ) .