Cocoa butter can seem a little tricky to find because it is rarely called for in the home kitchen. It is essential to the manufacture of chocolate, but this task is rarely taken on by home cooks.
Cocoa butter is actually very easy to find, but you have to switch your thinking from "cooking" to "skin care." Pure cocoa butter is available in the skin care section of many supermarkets and most pharmacies. As long as it is labeled "100% cocoa butter," it should be fine for use in your recipe. We prefer jars that are also labeled "food grade," in which case there is no doubt that it is edible. Check the ingredients in any case, to see that there are no solvents, fillers, preservatives, or chemical additives.
You can occasionally find cocoa butter that was never meant for anything other than cooking, but in our experience sources come and go. A possible benefit of some of these never-anything-but-food cocoa butters is that they may not have been exposed to high heat in processing, which, their manufacturer's claim, makes them melt and blend with other ingredients more easily. We are not sure, though, that that claim is anything more than marketing talk. All the cocoa butters we have used have melted successfully in the microwave and softened nicely in a container in a pan of warm water.
We have found these never-anything-but-food cocoa butters to be considerably more expensive than food-grade cocoa butters sold as skin-care products.
White chocolate is essentially a combination of cocoa butter, milk solids, and as much as 60% sugar. If you can't find cocoa butter, you could possibly use some amount of white chocolate and cut back on some of the sugar called for in your recipe. Without knowing the exact make-up of your white chocolate, however, you will not know how much cocoa butter and how much sugar (or how much milk, lecithin and/or vanillin) you are adding to your batter. Depending on the recipe and your guessing skills, it may be a successful substitution and it may not.