Sligro or Makro might have it, but you need a KVK number to shop there. A polish supermarket in Enschede had bottles last month, maybe they can order a keg for you too. Worth asking around those type of shops.
I doubt Sligro or Makro has it. You'd probably be better of trying either the Polish supermarket, there's also a Bulgarian one Gronau if I'm not mistaken or try Beer shop, not many around and they're usually a bit more expensive than the supermarket but the variety is way bigger, I'm pretty sure they could order some for you.