You did not mention why you are interested in brannerite, so I am assuming you want it as a collector's specimen and not a microscopic crystal.
Unfortunately, there is no definite source. However, there are some possibilities:
- Try the Handbook of Mineralogy: http://handbookofmineralogy.org and more specifically the brannerite page. You will fine there a more detailed list of localities for that mineral.
- Another option is to run a Google Scholar search for brannerite. You can find scholarly papers dealing with brannerite, and occasionally they also mention where they found the mineral or how they obtained it. Notice that in the specific case of brannerite, there are many articles dealing with brannerite structure. This may not be what you're looking for. There is also the problem of accessibility - unless you have an institutional or personal subscription to these journals, you may not be able to access the full article. However, sometimes this information can be found in the freely available abstract.
- Go to Rocks & Minerals magazine and run a search there for brannerite. They may have some articles dealing with localities.
Regarding your second question about geological features, the features may be mapped if they are big enough. This is probably true for granitic gneisses, may be true for pegmatites and conglomerates and unlikely for veins and placers. This will not help you though. Even though the mineral appears in let's say gneisses, 99.9% of gneisses will not contain this mineral. Of those 0.1% that do contain it, only a fraction will have it big enough so you will be able to see with without a microscope, or any larger than a speck of sand.
Finding rare minerals is indeed hard. If however you do indeed come across brannerite, I would like to remind you that it contains quite a lot of radioactive elements so don't make jewelry or good luck charms out of it, and washing your hands after dealing with it is usually a good idea.