Reddit mentions of Pokemon Starters Allover Print Men's Pajama Pants (Medium) White

I found these ones that are exactly the same on Ebay, but they're used and there's only three days or so left on the auction. Bonus: Looks like the same carpet as Digibro's old house. 99% chance just a coincidence, but that 1% means you might be buying the actual pants from the video.

I also found something pretty close. It's the Gen 1 starters, but there's no black background and it's uniform all over instead of having big pictures of them on the left leg. In my opinion it looks a little better.

I also found this one that's a little bit closer, but it's all the Eeveelutions instead of the starters and still isn't different on the left leg.