Of course, you know that selenium works in tandem with... iodine. I wish there were a punchline in there, although even just mentioning iodine any more may be its own punchline.
Selenium isn't something I think I need to supplement, since it seems to come from a variety of plant and animal sources. There is some discussion of selenium being depleted from US soils, but it appears that selenium content is pretty solid in large sections of the country, including food grown around your area.
You do have to be careful not to overdo it. If you want to boost selenium, sounds like a few brazil nuts every now and again is a fine option.