My printer is one of the Pixma range. I'd never consider paying for Canon cartridges when you can easily buy much cheaper equivalents.
Tesco are (even for their own brand) very expensive for inkjet cartridges. The best of the supermarkets is probably Morrisons. That's where I get most of mine from. (There should be a guide, alongside the cartridges, to tell you which number you require. Additionally, there should be a list of printers on the packaging of the cartridges).
A brand worth looking out for, on the high street, is 'Print Aid'. You can often find their cartridges in the smaller retailers who specialise in printer consumables. They're an excellent brand and usually sold quite cheaply. (It might also be worth checking out market stalls to see if can find them. I know you can buy them from Clacton Indoor Market).
There are also hundreds of online sellers of printer inks. The quality can vary greatly, so it's best to ask around for recommendations. If I was buying online, I'd probably opt to use InkCycle. They're far from being the cheapest supplier but their inks are excellent and the service is wonderful. (You can phone them late in the afternoon and your ink will always arrive the following day):
http://www.inkcycle.co.uk/inkjet_cartridges.ph p?cPath=270
Chris