In line headphone volume control on closeout at Radio Shack
I found the ACCURIAN in line stereo 1/8th inch head phone volume control on close out for under $3 at my area Radio Shack. I have one of RS own brand, but it has a large opening around the volume control thumb wheel, and dust does get in pretty easily. The newer RS ones look like they may not have such a large opening. This ACCURIAN seems quite snug. Stock number is 420-0136, and RS seems to have added a zero to each of the two numbers. [I forgot exactly where]
I like these because they have very flexible leads, and are easy on the jack in the radio. They can be adjusted to put a little load inline with each channel when driving a mono amp with an adapter which shorts the channels together.
Directly from Koss or on Ebay you can find a pretty nice looking one for around $10