Hard and brittle fat as cocoa butter alternative

Such a thing doesn't exist. Cocoa butter is absolutely unique in that respect, to the point where even thinking of cocoa butter only is not sufficient - only two of its six crystal forms give you the desirable texture, that's why we temper chocolate. With anything else, every halfway mindful eater will easily recognize that what they are eating is not real chocolate, and has a quite different texture.

The closest you can get are commercial replacements. They are sold under names such as "baking chocolate", usually in the baking aisle. If you have never used them yourself, you probably know them from eating commercially prepared foods with them, for example they make up the chocolate glaze on ice cream, or on the packaged marble cake in supermarkets, or on other products such as jaffa cakes. They contain a high percentage of real cocoa powder, the fat is specially formulated vegetable fat, and at freezing temperatures, they do have a satisfying snap (see the ice cream case), while at room temperature, a thin layer of them is somewhat comparable to a dry ganache. They do work well as a cheaper alternative of adding a bit of chocolate to baked goods, but not for pure chocolate work.

The cocoa fat somebody else mentioned is hard at room temperature, but has a completely different texture. It also has the unique quality of melting at body temperature, taking its energy from the mouth. This is why it is used in "ice confections", candies which cool down your mouth when you hold them on your tongue. Beside this very noticeable effect, it has no snap, and while it does make solid confections, they won't be mistaken for being made with actual cocoa butter.

The third option you can use, if you are trying to save money and not to work around an allergy, is to work with the cheapest unfilled chocolate bars you can buy. They use a mixture of cocoa butter, heavier fractions of vegetable fats, and special emulsifiers which make the texture somewhat closer to real chocolate. They do have a slightly different, waxy consistency, but in some parts of the world (e.g. the USA), this is what people actually associate with chocolate, due to the use of these emulsifiers in the most popular brands of chocolate bars. You can melt and shape these bars reasonably well, as well as make ganaches and similar with them, and the taste and texture is not worse than when you bite into the bar as-is. If you like the taste, it should be a viable option for you.