Trainer tire for Specialized mountain bike with a Cycle Ops
You can use a tire that is narrower than the regular one for use on a trainer. You can even use one that is nominally too narrow for the rim as the tire is not supporting any weight or resisting cornering forces. As along as the tire is not narrower than the outside of the rim it will work.

Also, if you can’t find a trainer specific tire you don't have to use one, any road tire with no tread or a light tread pattern will work.