This gets asked every week. My favourite company is choice, they are good cartridges and the price you see is the price you pay as postage is included. click here some have also mentioned bigpockets click here
I see no difference betwen the Choice carts and the original Epsons on my printer, you may find a difference in yours. The cheapest cartridge is not always the best but theres certainly no reason to buy the originals as they are too dear.