Fp4+ is not a direct substitute for PX125 but it is a great film. Pan F has a bit more inherent contrast but can look extremely good if used under the right light conditions (not too contrasty). Grain wise FP4 and PX125 are about the same PX creates smoother tones in my opinion but that's about it. Since you use 35mm film another option would be APX100 which is a bit more grainy but has a very special look especially in the midtones. For convenience sake just get the FP4 and develop it in ID11/D76 or X-tol.

Dominik