Ответ пришел из Хельсинки:
1. At the moment you can find toast bags and grilling bags from all Prisma supermarkets. (In the supermarket the bags are usually situated in the same shelf than baking papers, aluminum foils etc.)
2. One pack contains two bags. The price is approximately 3,70-4,50 €/pack (grilling bags are more expensive).
3. You can use the bags several times (usually 10-20 times) and they are washable in dishwasher. It’s quite difficult to say how many times you can use them because if the bag is in direct contact with toaster’s heating element it’ll be broken.
4. The both sides of the bags are same material so you can turn the bags inside out. Actually it won’t be broken so easily if you turn it. That’s because the corners of the bags are not so sharp then.