It sounds like you are referring to the "coolant crossover pipe" by the alternator, correct? If so, your local Ford parts department can order the o-rings for you (if they don't have any in stock).
If your local Ford parts dept isn't very competent and looks at you sideways when you inquire about said o-rings, try giving Steve at Tousley Ford in MN a call (Google for his info). Or, try any of the other "mustang friendly" Ford parts guys who cater to enthusiasts on the 'stang msg boards...
G'luck.
edit - found the part number via our handy search function:
391533-S100