any old 1-1/8" fork can fit any modern frame with the right headset bottom assembly.
what is the travel length of the current fork? for a 100mm travel 29er fork, you'll probably want a 480mm or so fork. for a 120mm fork, it might be more like 490-500.
how much are you willing to spend? name-brand carbon fiber rigid forks can cost hundreds of bucks, but a nice steel one can often be found for under $100. lots of used Surly forks out there.
What size is the front axle? depending on the model and year of the bike, it could be 100x9 (conventional quick release), 100x15 (conventional thru-axle), or 110x15 (modern boost spacing).