Hi Tom,
When you say a two channel mixer do you mean that you want it to have two mic channels? For example, the photo below is of my Behringer 502 mixer, it has 5 inputs, but only one of those is a mic input and that doesn't have phantom power.
I should have dusted it before photographing it.
Without phantom power I'm limited to using a dynamic microphone such as this:
This mixer is the cheapest I know of, but is not USB and I don't know of any cheap two channel USB mixers. Behringer do make a USB converter that converts an analogue stereo feed into a digital signal and it goes for about £30:
U-CONTROL UCA202
This could be plugged into the output of the mixer above to give you a possible solution.
All the best,
Chris