Omega-3 is not produced in the body, but must be part of the dietary intake. The traditional source of polyunsaturated fatty acids is cold-water fish such as salmon, sardines, tuna etc. However, modern diets are generally lacking in sufficient amounts of these. In order to get enough of the right PUFAs - specifically, to get omega-3 fatty acids and not omega-6 - people need to eat at least two meals of the right fish each week. These are not always available, and are becoming much more expensive.
Simple dietary supplements are available with high levels of omega-3, especially with the compounds that are known to be most beneficial for ADHD in children, eicosapentaenoic acid (EPA) and docosahexaenoic acid. One example is Zoom. For adults there is Vayacog, which is an equivalent source of the omega-3 compounds that are most suitable for older people.