hi hoshisato

im not in europe but was able to get the ingredients there when i was in france.
the coffee was easy it was found at the large grocery chain marmut ( i think that was the name )
the cheapest i could find was "el gringo" instant coffee, it worked great !
the washing soda and vit c i unfortunately bought at a pharmacy i wasn't able to find them locally .. BUT
i know now you can convert baking soda ( sodium bicarbonate ) to washing soda by purging the moisture out
in an oven. spread it on a cookie sheet and heat for a while at a low temperature.
i can't recommend where to find the vit c. maybe someone locally can suggest where to find it,
i know in some places in europe ( northern europe ) the vit c sometimes has other things mixed into it ...

maybe reinhold's caffenol blog can help direct you too
good luck !
( and have fun )