The Leech Dragon, so named for its round, leech-like mouth, is by far one of the creepiest and most vicious dragons. Part of the reason for its infamy is its ability to exhale toxic and very flammable clouds of methane gas. These relatively small dragons, (about 5-6 feet long and very thin), congregate together and can rapidly turn enclosed or poorly ventilated areas into lethal gas chambers. In addition, these dragons can create sparks by rubbing their teeth or scales together, which ignites their methane breath attack and creates a large and very fast moving fireball.
So here is my question. Where does the dragon get the methane and how does it release it? Alternatively is there some other gas that would work better/be easier to obtain?