Genesis, Lunar biome. Take an anky, any normal looking rock you smack will have a small amount. Anky can also get ele sharts and Ambergris from the green glowing lunar rocks. Spawn Lunar East, in one of the craters there's a small cave with loads of rocks, that leads into a chamber with water, fish, seekers, featherlights and occasionally a Karkinos, you can get loads of gas in there without the sun radiation or meteor strikes.