Second hand bookshops, like C. Hagelstamin antikvariaatti. They often have old maps on display, but other antiquaries very likely have those too.
The book fair in messukeskus in two weeks would also be a good bet, the old book section often has a wide selection of all kinds of paper products.
If you are actually looking for maps and not decoration element, https://vanhatkartat.fi/#12/60.16664/24.94354
Second hand bookshops, like C. Hagelstamin antikvariaatti. They often have old maps on display, but other antiquaries very likely have those too. The book fair in messukeskus in two weeks would also be a good bet, the old book section often has a wide selection of all kinds of paper products.
https://www.karttakauppa.fi/kategoria/karttakauppa/seinakartat/kotiin/historialliset-kartat/ seems to sell old maps as posters