In your search for human selenium capsules you on Puritan or Swanson's vitamins you probably noticed the dosage is in mcg.  You said you wanted to supplement  2 mg per horse.  1 mg = 1,000 mcg.  Do the math on cost and number of capsules you would need to supplement.   In my search,  I didn't find equine selenium sold in capsules.    A work around would be for you to purchase empty capsules and load them yourself with 2 mg of an equine selenium product such a Uckele's or Platinum  Performance......or consider putting the individual portion in a small plastic container with a lid.