Are you wanting to go old school regular or the new CE2 cartos?
The two cheapest places I've found for regular cartos:
Strictly E-Juice : They have cartos for $1.45 apiece or $7.00 for a 5 pack. This is about the cheapest I've seen them. Plus they have free shipping for orders over $10. I can get about 2 days use out of one. These do not clean well to be reused though. It's also tough to get them because they sell fast and often out of stock.
Liquid Express Blank Cartos These run $8.00 for a 5 pack and you pay shipping but I've had good luck recycling old ones for extended use.
Won't need a syringe for those.
The only place I've bought CE2 cartos is from other members here in the classified section.