That’s a standard mountain bike tire size. You can get a bigger one that will be a bit nicer on trails. I would probably not get anything smaller than 26x2.125. There are a lot of tires available but you might want to check this thread for some of the popular options.
Keep in mind that your uni isn’t nearly as bombproof as a real MUni. Your rim is probably pretty narrow, and so you won’t want to put a 3" tire on it (it will fold with lower pressure), but you might do well with a 2.3-2.5.