^ This.
I've bought the battery packs from the 99p stores and they don't last well at all.
Most of the local supermarkets in my part of SE England / N. London sell both LR44 and CR2032 - I've found them in Tesco and Asda - also picked some up in my local Wilkinsons store once - they were fine too. Failing all those - Currys, Argos, etc. - and Amazon of course - but you will need to do a bit of price checking / comparisons - the store prices change regularly and sometimes Tesco is more competitive, other times you can get them cheaper in another store like Currys if theres's some kind of special promo going on.
Edit: The other thing is to try to buy recognised brands like Duracell / Energizer / Panasonic - they do cost a bit more but they are more reliable and last longer.
