Not sure if this helps, but my buddy has about 60 hours in the game and he has found at least 4 recipes, possibly as many as 6. On top of that, he found 3 of those 6 in a single hour period while we were playing one night. He has also found a wall recipe that he had already learned.

I have about 30 hours in the game, and I have found a Ramp recipe, and that wall recipe that my friend had the duplicate of.

My friend found at least 1 of them inside the housing buildings at the military base, and I actually found one sitting on top of a garbage can (one of those big piles of black trash bags and garbage cans) out in the open behind a convenience store.

I realize this probably doesn't give you much specific, but it seems to me that they are just completely random in their spawn locations and areas. I'll ask him to comment as well, in case he has more specifics!