According to what I can see, your sub requires a 7.2v 1300mah battery (ies).
It really depends upon the size of the battery compartment as to what is best.
That is, if your compartment will hold a 7.2v 1800mah battery then you would have a longer run time.
The Tower Hobbies part number for a 7.2v 1300mah battery is LXBEZH.
What is important is the 7.2v (7.2 volt) while the larger the "mah" number the better.
"mah" is short for milli amp hours (current) and 1300mah would mean you can use 1300ma for 1 hour.
Hope this gives you a start.
Just one more wire!