The best type of mustache brush will be one targeted toward the type of mustache you have. Most mustache combs and brushes are very small and feature fine metal or plastic teeth. If you will be styling your mustache in a particular way, it's a good idea to use a brush specifically for the look you desire. For instance, if you have a handlebar mustache, you should use a comb with very fine metal teeth in order to style it properly.
If your mustache is very thin and short and the hair is fine, you may not need a mustache brush or it may not matter which type you use. As long as you keep it adequately trimmed and groomed, thinner mustaches generally avoid the unruly or tangled appearance that longer ones can have. If your mustache is very thick, long, or coarse it may take some shopping around before you find the mustache brush that works best for the length and style you are going for.
There are a variety of ways you can go about finding a mustache brush. You can likely find the right kind by shopping at a barber's supply store or by speaking with your barber to discuss options. You may also be able to order online from a retailer specializing in mustache maintenance. There are often items designed specifically to groom certain types of mustaches, so you will want to choose the one most suited for your needs.
Many mustache trimmers also come with combs. This allows you to trim and comb at the same time, although these are often intended for thinner mustaches. Thicker mustaches are best suited for very fine-toothed combs, usually metal, which are able to push hair into place more efficiently. They also work better with wax and other styling products that are designed to help control long mustaches.
Price is another factor to consider. There is generally no need to spend much money on a mustache brush, although buying one which comes with additional supplies may become pricey. You will likely need various combs, trimmers, and brushes to keep your mustache looking its best during different stages of growth or if you change the style. Buying them in a kit is often a less expensive option.