Radio Shack or a Computer Store (Some networking connections are BNC).
BNC is used by some high-tech medical test equipment, so you could try a medical equipment dealer. I assume these would be high quality (but expensive).
sean--hate to disappoint you, but the geometry of a RCA cable, w/ the center pin having greater length than the return, prohibits it from ever being 75ohm. RCAs can never be 75ohm. BNCs can and often times are (though some come in at 50).
do search on audioasylum > cable for 75ohm RCA if still you doubt.
fwiw: the canare RCAs, while not a true 75ohm, are still the best digital RCAs available.