The best (by which I mean easiest) place to get a pulley for one of these engines would be China... Aliexpress is probably a good site to search; also Alibaba and Banggood - the former is more for wholesale level orders, but there are companies who will handle single-item orders and courier shipping. Aliexpress and Banggood are more aimed at consumers, so there's not as much stuff available, but it's usually easier to buy than on Alibaba.
Shipping to Europe from China gets expensive with weight; if you can wait, though, surface mail is still quite acceptably priced, if slow.