That'll open up a can of worms. Any sus fork will be considerably taller than your rigid, which will change the geometry and handling of the bike. Some worry about the extra stress on the headtube/downtube joint.
An educated guess is that you need to look for a 1 1/8" threaded fork, unless you want to convert to threadless/aheadset. Make sure the threaded section is long enough to fit the headtube length of the intended frame.
Oh, should be for 26" AKA 559 mm wheels, and have bosses for rim brakes.