I think this is what you need. I have a couple of these on my DACs and they work quite well.
http://www.google.com/products/catalog?sourceid=navclient&oq=&rlz=1T4DMUS_enUS305US305&q=RCA+to+BNC+adapters&um=1&ie=UTF-8&cid=10546853561807035384&ei=6AahS7fSB4zsswOvgLHTCw&sa=X&oi=product_catalog_result&ct=image&resnum=2&ved=0CBwQ8gIwAQ#
http://www.google.com/products/catalog?sourceid=navclient&oq=&rlz=1T4DMUS_enUS305US305&q=RCA+to+BNC+adapters&um=1&ie=UTF-8&cid=10546853561807035384&ei=6AahS7fSB4zsswOvgLHTCw&sa=X&oi=product_catalog_result&ct=image&resnum=2&ved=0CBwQ8gIwAQ#