7dayshop as mentioned above. Every cart I bought always seemed completely genuine Canon, not a hooky copy.
Amazon does sometimes do the best price, but it's worth using a price watcher like camelcamelcamel and selecting sold by Amazon only (who knows what 3d party sellers are off loading?!?). You'll get an email alert when your desired price is reached and you can then put an order in.
^ These are the two I bounce between for my small electronic consumables and ink purchases.
Cartridgepeople look interesting, Jersey based.
Staples, surprisingly weren't that expensive.
Advantage-online maybe a little pricier than some.
inkfactory
As far as I can tell, Canon Chromalife100(+) ink doesn't have an expiry date. I doubt you'll see one on a package anywhere. I seem to recall emailing about older CLI-8 carts and they said they were fine as long as they were kept in the packaging, in a cool environment out of the light. Common sense stuff. So really, if you won't mind the outlay, you can always buy a couple of each cartridge and have a spare supply in the cupboard. The only annoyance with this approach being that the printer will inevitably fail at some point and you'll be sitting on a sizeable investment of now useless ink that won't fit a new printer model. Anyone want any CLI-8 carts by the way?
Don't buy ink in a supermarket or department store unless you enjoy a 10-30% markup! Online is the way to go.
Avoid Valueshop and all of it's linked names. It's gone bust essentially, just look into the accounting. Used to be fine years ago, but don't buy anything from them or subsidiaries unless you want to get rid of your money and don't like charities.