You can either do a little DIY and put modern NIMH AA cells in your existing pack or, if you don't fancy that, then E Bay is your friend:
http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=canon+NP-E2&_sop=15
The more modern NIMH cells will charge just fine on your existing charger but they will take longer due to their much higher capacity.