Currently there is no issue with mainstream 26" tire availability online. Regardless of your discipline and riding conditions, you should be able to find a suitable tire. You may not find them in local bike shops but you can certainly find and purchase them online.

However, there are an ever-growing number of new tires that are not being produced in 26". Some day you might not have the tire you want. Still isn't a big deal compared to riding 650B anytime before 2014, or 29ers before ~2010. There are plenty of 26" tires.