OCBD = Oxford cloth button-down. Oxford cloth is a bit heavier and coarser than say a broadcloth shirt. Google it up for more information. Button-down refers to the collar buttoning down to the shirt.
You can find them pretty much anywhere that sells menswear. My favorite off-the-rack ones come from Brooks Brothers.
If you’re on a budget and are “regular” sized in your proportions, then I’d check out Lands’ End Canvas. Their collars are a bit flimsy and small for my tastes to be worn with some ties, but they’re fairly cheap and often on sale.