Since they are so hard to find, making your own might be your only option. Unless you want to wait years. And you don;t have to cringe at cutting into such a hard to come by piece to add the vent and fins.
Couldn't tell you the last time I saw one anywhere. Not something I'm into, but one of those things I look for here and there due to the rarity. I agree with making your own, so you don't have to cut up a good one *if* you ever find one.