You can get Federal Pacific replacement breakers from any professional electric supply company, or have them ordered for you through them.
There is a single company that bought the rights to manufacture FP replacement breakers in the US (whose name escapes me at the moment) but they are manufactured to appropriate standards and operate properly.
They are not cheap, however.
I recently purchased a standard 20 amp breaker and the price was nearly $40US for this one single pole breaker.
I guess that is the price of not having any market competition for this particular product.