Well it depends. What charging equipment do you have and how many volts do you need?

There are loads of places to get nicads but be careful. I went for the cheap option once and bought some nicads from maplins. They barely held enough charge to last a fight and eventually cought fire.

I then bought some packs from overlander and they have been perfect. Was around 50-60 pounds for 2x 12volt 3.7 amp Nimh

So I suggest buy well and buy once

obv link
http://www.overlander.co.uk/http://www.overlander.co.uk/ (great but not a cheap option)

I am sure there will be more links for cheap nicad battery packs to follow.