Sounds as if you have a standard mountain-bike size rear wheel. These are labeled 26" and have an outside diameter of 559mm (whereas 700c road bike rims - the same as "29-er" mtb rims - have an outside diameter of 622mm).
I'm not sure if your goal is to replace the entire wheel, or to rebuild the current wheel (re-using the hub and spokes) with a new rim.
Either way, there's lots of options out there. If you rebuild using the same hub and spokes, you need to get a rim with the same Effective Rim Diameter. Basically, rims with a deeper sidewall require shorter spokes (pretty obvious if you think about it, but the the sort of thing most people would think of unless they'd run into the concept before). You can probably get away with a rim that has a 1-2mm difference in E.R.D. in either direction, but that's not guaranteed.