As james_w in the uk i'll add that I used this battery for mine : http://www.componentshop.co.uk/6v-4-5ah-sealed-lead-acid-battery.html(they also carry a 5ah battery of the same size and price: http://www.componentshop.co.uk/6v-5-0ah-sealed-lead-acid-battery.html (i'm sure that one cost more when i brought mine) )

Currently i have just soldered a couple of croc clips onto the battery and i just clip those to the contacts in the portable (I haven't got around to recelling the orig pack yet)