Funny a thread about this came up. I'm in a similar bind over Cthulhu d20. Anyone know of a cheap ( = cover price) location I can buy it from? Even pdf?
The cheapest place I've seen it today is at abebooks, $55 + 3.50 shipping. I've also seen illegal copies of the pdf floating around p2p networks ... but downloading that way is totally against the law (and therefore something that I would advise against).
The sanity rules are almost word for word found here. All you have to do is replace Knowledge (Forbidden Lore) with Knowledge (Cthulhu Mythos) and you're good to go. (Although it may just be Cthulhu Mythos... it's been a long time since I read that book.)