I really have no idea here, but wouldn't any bass knob that uses a phone plug style cord work? I haven't looked, but I imagine they are all just a simple potentiometer.
If any would work, I have an infinity one that came in the car that I just bought, and I have no use for it - its black with a little knob and has a phone cord style plug on the side that plugs into the amp. I'd give it to you for the price of shipping if you want it; or since I'm in fort worth, we could meet up somewhere near me and you could try it out to see if it would work