I got it from the Private Key page on the Bitcoin wiki. I don't know whether the number is right or wrong, but feel free to update the wiki (perhaps it should have a citation, even?) if the value there is wrong. I apologize for not checking my sources more thoroughly, or at least citing where I was getting my info from.

In any event, it's rather unlikely that a random 256-bit number won't be in the range. If somebody gets heads a ton of times in a row, they may want to double-check the upper bound before using it. (Or, perhaps more likely, check that their coin is fair…)