Ilford Perceptol is a decent substitute for Microdol-X. And if you can't get that you can always mix your own D23:
Distilled Water (125 degrees F) . . . . . . . 750 ml
Metol . . . . . . . . . . . . . . . . . . . . . . . . .7.5 g
Sodium Sulfite (Anhydrous) . . . . . . . . . . 100 g
Cold Water to make . . . . . . . . . . . . . . . 1 ltr
or Grant Haist's formula for a salt containing developer that he attributed
to Egar Hyman.
Metol 5.0 grams
Sodium Sulfite, anhydrous 100.0 grams
Sodium Chloride 30.0 grams
Water to make 1.0 liter
They all work well with Plus-X, just from my own experience.
Marty