yes, you need an adapter with 2 mono 1/4" male jacks on one end and a stereo 1/8" plug on the other end. you could get an cable with all 1/4" plugs, 2 mono and 1 stereo, and use a 1/4" stereo to 1/8" stereo adapter.
i doubt that your local radio shack has an adapter like that but it's possible. they seem to have everything in stock except the precise thing that you need.
this is what you need:
http://www.amazon.com/gp/product/B0002CZZ0G
i would look for something with non-molded connectors though.........something that you can resolder if (when) the connections break.