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).
I had the opposite problem and found some RCA to BnC adaptors at my HiFi store however Radio Shack was recommended to me by several people for decent adaptors.
I got RCA to BNC adapters from Radio Shack, as Sugarbrie also mentioned. They have a nice gold plated adaptor which sold for about $4 last year. The non-gold plated ones cost less.
These adapters need to be 75 ohm. Rat Shack doesn't say what theirs are but I'd bet they are the more common 50 ohm. Pay a little more from someone like Michael Percy and be sure you are buying 75 ohm adapters. Don
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