+1, the easiest will be to find a battery, & do some soldering.
Batteries: 'bout 2 x $15
fine soldering iron: $20
soldering lead: $5
Perhaps some soldering grease: $5
Total $60; though most people have some form of soldering iron at home,or can lend it from a friend, so don't need to pay extra for them.
It would be better if the battery had an internal USB port plug that would fit eg: this (or a smaller) type of cable:
