Originally Posted by
Kapusta
Mostly 29". The rest 27.5". At least for non-BSO bikes.
You can still get some good 26" tires, though not the selection of the other sizes.
There's still a vast selection of 559 tires, since various bikes still use them. Maybe specifically offroad ones may not have a large variety, bu as a whole; there are choices. I haven't really looked online, though, I just get whatever the bike shops have.