Ferminx: Parts Connexion or get Moon Audio Silver Dragon. In both cases it'll be expensive.
I was at my local DIY parts store looking to make a special TRS to RCA adaptor for my ULN-2 (it must have the ring not connected) and found a TRS plug that PERFECTLY fits those small, cheap generic RCA sockets. I could literally screw them in place inside the plug! I only had to bend the tab for the wire on the inner ring 90 degrees. Having that actually made it easier to drop the nut in and screw it tight.