Poster board? Your talking about the stuff for the canopy and not the actual foam board right? It's just a thicker paper. Here's a link to some on amazon -
Royal Brites Poster Board White, 22 x 28 Inches, 50-Sheet Case (24301)
https://www.amazon.com/dp/B000RSZWDK/?tag=lstir-20
There are a lot of others listed on Amazon, you don't have to by this exact one. Over in Sweden I'd assume you have office supply stores or maybe Walmart type stores, they should carry poster board as well.
The actual air frame itself is made of foam board. You can get that from Amazon as well and maybe your local office supply. But be aware, there are many different types of foam board with varying degrees of weight and strength. Here in the states we use Dollar Tree foam board or others like Ross foam board from Walmart.