Any "Standard" (only in the US, now - so it isn't really a "standard") ruler is a N. American O-scale (1:48) ruler, as 1/4" - 1 foot. Even the free ones. However, they don't help you in Britain or Europe, where O "scales" vary from 1:43 to 1:43.5 to 1:45, maybe more, to my understanding.
A metal one is better, or at least prettier, but I lost mine years ago and just use an old wooden give-away ruler for building everything. Still 1/4" to the foot.