I doubt you would find a Mod tire in stock at a bike shop, but most should be able to order it for you no problem. Check around until you find one that will. Besides LBS’s slightly higher prices, but no shipping fees is usually a better deal than online.
Onza sticky fingers has 20X2.4 & 19X2.5
Monty Eagle Claw comes in 19X2.6
Spencer got that Twenty-G * 20X3.45" * tire (It’s really like 2.5"), and in order to fit it in his Bedford 20" frame he had to shim the bearings.
Tread life depends on the surface you are riding on, and how you ride. I have difficulty pedaling smoothly and I put a lot of wear on the part of the tire on the ground w/ left pedal down.