Everything said so far is correct. Also take into account how much recoil the gun the gun will have and your experience level. As the recoil increases the direction of the grain becomes more important. Lots of figure also weakens the wood. Burl is not very strong at all. Highly figured wood is also harder to work with. The strongest wood plain and straight grained. It is also the easiest to work with and the cheapest. Plain walnut is stronger than plain maple. Highly figured maple is stronger than highly figured walnut. Sugar maple is very hard which makes it good for carving. Red maple will usually have better figure, but it is softer. Highly figured sugar maple will be significantly more expensive than the same figure in red maple. With walnut you have American black walnut and European walnut. Those are two different species of wood. black walnut is open grained and European walnut is closed grained. Claro, English, French, and Turkish walnut is all the same species just from different areas. Each is slightly different due to different growing conditions. Bastogne walnut is a hybrid of black and European walnut and is very strong and dense.
Or, do what I suspect the big majority do is order from a reputable vendor and hope for the best.
If you are working with a good dealer you can tell them what you are looking for and how much you want to spend. You will likely be happy with what they send you. It is best to deal with someone that specializes in gun blanks. A sawmill may have that perfect piece of wood and the price is likely to be lower, but you will need to pick it out yourself. Most blanks are sold by the grade. The high end blanks are going to priced individually and the dealer will usually provide several pictures of each one. If you are getting into that stuff expect to spend some money. A plain blank is not very expensive. A nice maple blank can get expensive. Black walnut is more expensive. European walnut blanks can easily cost thousands of dollars.