Martelli cutter is the best I have ever found. They have a handle that curves and fits into the palm of your hand. The pointer finger rests on top of the blade area so makes cutting very strain free. I've probably tried them all and always had a sore wrist when cutting for any length of time.
For replacement blades, go to Harbor Freight and order carpet cutter replacement blades. 2 for approx $1.99 They fit the cutter and seem to last way longer than the regular cutter blades. Hope this helps.