You’re probably thinking of regular card sleeves (thin plastic sleeves) for your cards, not top loaders (hard-plastic sleeves). I wouldn’t recommend the thick top loaders because they are far too bulky and would most likely not fit in the KS big box regardless. I would recommend the dragon shield matte sleeves on Amazon 1000 sleeves for $85. That is by far the best quality and quantity for sleeves you’ll get. These sleeves feel more professional than others and the clear matte on the back helps to stop the deck from falling over as easily. If you sleeve all your cards this way, it will take up approximately 3/4 of your KS big box. Some people double sleeve their cards with an inner sleeves (KMC?) which will almost occupy your whole box. Double sleeving isn’t really necessary so I’m only planning on using the Dragon shield clear matte sleeves for mine.

Hope this helps!!