Ukraine can shut down the gas in the far west and still maintain flow for use inside the country. Check the map.
The source of the gas is still Russia. Russia has shut off Ukraine in the past for unpaid bills and for embezzlement of the gas, which had the side-effect of shutting off Europe as well.
Ukraine has had talks with Azerbaijan and Kazakhstan about a pipeline under the Black Sea to bypass Russia which would give them a non-Russian source of oil and gas. It would have come ashore in Crimea, though, if I'm not mistaken.
I don't think it ever got off dead-center, though. They did build a pipeline out through Turkey, though.