Originally Posted by SkyDog75

Pretty much any Shimano 7/8 speed triple mountain bike derailleur (Altus, Acera, Alivio, Deore, etc.) should work, so long as:

1) The derailleur isn't designed for a chainring smaller than yours. If you've got a 53 tooth chainring, don't use a derailleur designed for a max 42-tooth ring.

2) Its cable pull direction matches your frame's cable routing. If your derailleur cable runs along the top tube, then down the seat tube, you need a "top pull" derailleur. If your derailleur cable runs along the downtube, then up from the bottom bracket to the derailleur, you need a "bottom pull" derailleur.
and its cable is designed to be pulled from the right direction.

...and so long as you've got your crankset matched to a bottom bracket whose spindle is the appropriate length. If the spindle is too long or too short for your crankset, the derailleur might have a hard time reaching all your chainrings.