That isn't a toploader. That is a card that has been graded by PSA. PSA is a company that officially grades the conditions of cards so that collectors can be assured that the card they are receiving is of a high quality and has been well cared for. For the item you linked, that card had been rated "10 - Gem Mint" (the highest possible rating) which you can see in the upper right. If you want to get an official case like that for a particular card of yours, you can submit it to PSA for grading. Information about that can be found on their website here.
I would imagine that it's probably rather difficult for the average consumer to get cases like those used for grading because it would lead to easier counterfeiting of graded cards. You are correct in that PSA very likely does not manufacture the cases themselves, but I would not be surprised in the slightest if they (and other grading companies) keep strict control on the distribution of the holders they use.
But I'm pretty sure PSA doesn't manufacture their own card cases.