I may not live in the Netherlands, but the place I hold league sells toploaders. Around here most hobby shops that sell trading cards will sell top loaders and sleeves. Sometimes toploaders can be found near the trading cards in convience stores such as Walmart and Target.
@Juliacoolo: The Walmart I work at sells them.
I highly doubt you'll get them at supermarkets. You may find them in the card isle, but most stores don't cary them. I know for a fact that Toys-R-Us/other toy stores have them. Your local league may have them as well.
(BTW, I think this belongs in the general TCG discussion.)
You can maybe try Spellenhuis in Den Haag or Haarlem.
Toy stores don't even sell sleeves or toploaders as well the warehouses.
You only can try some Game stores.
Try stores that carry an assortment of collectible card games, preferably those who also buy singles from you. In Utrecht, I know theres De Joker (Where I keep my league), Never Never Land and Labyrinth carrying them.