I have looked on several sites and unable...
I have looked on several sites and unable to locate the green gas cannister for my optimus stove. Where can I buy this particulat cannister?
That's a standard camping butane fuel canister. You can buy them anywhere that sells camping supplies. You don't need to buy the Optimus brand.
Echoing to some degree what Pat said, anything with a Lindall valve will work. That would include other brands like MSR, Snow Peak, Jetboil, etc... It's always better to buy fuel on the ground locally in a retail store. Since they can't be shipped by air and are considered a hazardous material, shipping costs from online stores is really expensive. Almost to the point of doubling your costs in some cases.