I don't think it is worthwhile to buy online unless you have other item that will be shipped together. The shipping will be a substantial fraction of the cost. It is about $5 at Petco/Petsmart.
The Two Little Fishies clip is the best. In addition to having a very good suction cup, it floats to the top if it should ever come loose. No more lossing clips behind the rocks!
pppfttt...I have had 2 or 3 "aquarium clips" for nori and algae sheets. All of them were crap.
Best solution I found: use a suction cup with some kind of "arm" like one for holding pipe in place, and rubberband the food on it. Works great, and ya probably already have all of the parts right now in your house.
I have this really cool clip called a "Mag float". It has like a magnet with felt on one side and velco on the other. When it is empty, I can use it to clean the glass!