There's always Hatton's Model Railway shop. They ship to the US. The only thing I'd suggest is to buy in bulk, as it will be cheaper that way. And about your flex track, if you have 100 3 foot pieces, that is 300 real feet. 300 scale feet is 6 50 foot boxcars, which is somewhere around 42 inches long.
Modeling the LNER (and some Thomas stuff) in OO Scale.
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