I use this
Se-Methyl L-Selenocysteine, 200 mcg, 90 vegetarian capsules - Life Extension
I don't think I take it more than twice per week, 1 capsule only. I also take a multivitamin that has 100mg of Selenium in there, but again I take this also once or twice per week max. there is some concern with too much selenium supplementation, some studies show toxic levels achieved with supplementation, but with the plant version which is called selenomethionine.
I also eat 3 eggs every day and at least 1lb of meat per day, plus weekly liver, and I don't eat any grains that may interfere with mineral absorption, so I should get enough from food.
The best thing would be to do some labs and pull a Selenium RBC to go with higher supplementation in my opinion, even too much supplementation can cause side effects. I just take a bit extra every week to go with the Iodine, because too much Iodine with not enough selenium can be a problem. Plus who knows if the animals I'm eating are ingesting enough Selenium to begin with, because we do know soils are depleted of minerals now days.