How many bolts do your cranks have? You'll need to match the bolt circle diameter with the chainrings you want (BCD).
After you know these two things, just search for the rings.
For example, my crankset is a 4-bolt shimano SLX mountain crank (triple) 22-32-44. The BCD is 104 for the middle and outer ring, and 64 for the inside ring.
So I would need to find 2 chainrings of the tooth size I want, in the 104 BCD and the other chainring as a 64 BCD.
Also, note that your front derailleur is matched to certain sized chainrings, so changing them around can sometimes affect shifting. If you change the middle from 36 to 44, I would venture a guess that your current front derailleur will rub on the middle chainring and not shift very well.
For a 26-44-48 you'd probably need a standard double front derailleur instead of a triple, since your're wanting rings closer to half-step gearing.
Last edited by TurbineBlade; 07-21-10 at 12:12 PM.