I bought this one right here: https://www.amazon.com/dp/B00NPZZ1JW
There are cheaper ones that may work just as well but I wanted one a little larger than those others so I didn’t have to sit quite so close. The reviews on this one were really good too. If you look around on lesswrong.com there are anecdotes of people taking the idea to extremes, rigging up a 100,000 lux system and sitting under it all day to work. I don’t know what difference that would make but speaking for myself, the light at that Amazon link gets the job done.