These seem to be proprietary to the hobby industry. Therefore, you have to pay top dollar for them. FWIW, Radio Shack has not been a place to buy electronic components for over 20 years.
There is a company that makes replacement plugs but I can't recall right now. After searching a few things I am pretty sure it was Cermark, but their page is down.
If you really want to order electronics stuff for real, go to a real electronics supply place with a great selection like Digi-Key http://www.digi-key.com/
They'll send you a free catalog and have one online too. Plus they will be more reasonably priced than any crap from RS. I still wouldn't hold out to get "those" plugs there. If you find a part number, be sure to let us know, ok?