mfbolton- you want to find a 4 ohm 2x10. Ampeg made a few models, including the BXT210M. But ANY 4 ohm 210 will work. I used a 4 ohm 1x18 with that amp, a 4 ohm 1x15 would work as well, but a 210 would be ideal.
unclewalley- you CANNOT use that 4 ohm ext cab with your 8 ohm cab, as your amp is 4 ohms minimum. You can use ONE 4 ohm cab, or TWO 8 ohm cabs. It's best for them to be identical, in any case.