Architectural Numbers by Weston is a company that lets you choose your font, material, and finish; they then custom manufacture your number and or letter combination. Choose from 41 font styles (including classics like Helvetica, Futura, and Times Bold) and 12 different finishes. (N.B. You can order in any size, but they say the standard is 5 inches high.)
Prices range by size and finish. From $15 and up.