It would help to know brand/model of printer, but I buy my ink from SwiftInk.com (self-appointed inkseller to the stars here on the AT forums) who often give us a good deal. Check the first page of the Hot Deals section frequently for his offers. If nothing there, check the Heatware.com site as he runs offers there too but not at as large a discount as he gives here.
But if cheap is your sole criterion, then eBay often has ink at the lowest price around. I look for the best quality at a reasonable price. At his regular prices, you'll get three tanks for the price of one OEM from Canon, when on sale here, it will be four tanks for the price of one OEM tank. Plus if you buy $50. worth or more you'll get free shipping.
.bh.