Qualcuno ha qualche consiglio su dove posso farlo a Helsinki?

I’d personally go with something like Vistaprint but there’s a Lohikari Oy store in Helsinki center that sells those roll-up banners, they don’t seem to list prices though.

Also you can try to google the words “Roll-up mainosbanneri” or “Roll-up mainosständi” to find a store that fits what you’re looking for.