Best thing to do is bring it into your local bike shop (LBS) and have them take a look at it. Generally you want to stick with the exact same tire (in terms of tread pattern and of course ISO diameter) Sometimes a 26" tire can have the same ISO diameter but will be a different tread which means it won't actually fit which I learned the hard way after blowing off a customers new tire several years ago. ISO 559 is ok in my book as is 650b but beyond that 26" can burn to death!! Such a pain in the butt to deal with.

Some shops do have those sizes in stock and some don't but generally most if not all can order them and install them for you. If you do it yourself it is helpful to know that while the tire might be able to hold a higher pressure your rim might not so be careful and best thing to do is use a hand pump and do it slowly checking that the bead is seated all the way around and probably not going above 50psi.