You know, (according to my dad) they have a counter inside them that schedules them to break down after a certain number of uses, when you use fake cartridges it makes the counter go alot faster so they break down quicker. I'm not sure if it's right though.
I usually get them via e-bay. And used ones I send to Tesco in their envelopes - you get clubcard points for most. If they aren't ones that Tesco take I sell them to someone on e-bay, but can't remember who as I haven't done this for a while.
Have you checked to see if it woudl work out cheaper to print online? Places like photobox you can do prints for 4p each when you order in bulk.
If you are prepared to spend the time, you can go through a cashback website like quidco, and take up all the offers of free prints, plus get paid for it (which cancels out shipping and a little extra you can then use towards paying for a cartridge for home use.)
I get Jettec brand cartridges for my Epson. Used to get their HP remanufactured ones for our HP at work. They are always really good quality and far cheaper than anything else. They are sold at lots of online stores. Go to Jettec.com and search for your local seller.