What size Mr. Tuffy Liner should I buy for my 24"x1.95" bike tires?
I'd recommend none. They're more likely to cause flats when installed improperly and increase rolling resistance and weight. You're better off purchasing Kevlar belted tires/ puncture resistant tires, or if you live in an area with thorns, thorn proof tubes. You should also make sure your rim tape is properly installed and your tires are properly inflated - with this, and the right tire choice, they should be unnecessary. 

See also this question.

Also, 1 3/8" is not the same as 1.375 inches when sizing tires, necessarily (see Brown's law of tire sizing). 

While I do not use Mr. Tuffy liners, I'd think the 26"x1.95" one with a bit of trimming is the way to go in this case if you still choose to use one.