I have never built a guitar, but have read several books about it and have spent many hours looking through luthier supply catalogs with the hope to one day make my own. I can say that it would not be typical to look for a board of the dimensions you list. The typical construction for the top and the back is to make them from two pieces of wood with a joint down the middle. Usually the top is made from a softwood such as spruce and the backs and sides from various hardwoods.
Rather than look for a board 3/16 x 26 x 24, you should instead look for a board more like 1/2 thick x 26 x 12, and then resaw it to form a "book-matched" pair. The 1/2 inch thick board would be sawn into two pieces 1/4 inch thick, and then "opened" as if it were a book. The result is two pieces with grain patterns that match almost exactly. One piece is essentially the mirror image of the other.
I don't know where to get the wood, but finding a good 12 inch wide board will be so much easier than finding a 24 inch wide board. Guitar woods are very often made from quarter sawn boards, so to get a 24 inch wide quarter sawn osage would require a tree over 4 ft in diameter.
Good luck, I would love to see an osage guitar!
