Most of us use 9V "DVD Player Battery" sometimes called "Walmart Battery" (although they don't sell them at walmart any more). They are sold under a bunch of different brand names, but they all look the same. A couple of years ago they were 4500mah, then 5400, now 6000 are pretty common and probably bigger. Prices range from $10 to $90. They will power a typical UA-5, or V3, or R4 for several hours. Here is one example.
http://www.amazon.com/Initial-RB-270-Lithium-Ion-Universal-Rechargeable/dp/B0002J9QXGNote that the Cakewalk recorder will allow you to record 8 inputs when you are hooked to a computer, but if you take it portable and write to an SD card it's only 2 track. That line "
Even record a mix of your set directly to the SD-card" is a bit misleading. If you only want to record 2 channels, there are plenty of other (cheaper) options, and if you want more channels then you need a computer and would probably use what you already have.