The postwar Lionel #91 breakers aren't too difficult to find, but one does have to keep an eye out for them.
One word of warning: If the adjusting knob on the #91 is turned down too tightly, the breaker will never trip. Sometimes you find them with melted cases. I'd steer clear of those because they were probably abused (by screwing the knob down too far.
I have been using the same ones for over thirty years, and have never had one fail.
I adjust mine by putting the biggest load I am going to run on the tracks, and loosening the knob until the breaker trips. Then I turn the knob back down 1/2 - 1 turn, and it's all set.
