I usually get my supplements from AmazonDE. But some supplements are a lot cheaper overseas.
iherb.com ships to Europe for only 4€. You can order for 26€ inluding shipping without having to worry about customs.
A few things where I found you can save some money: B12, methylfolate, vitamin d3.
Also you can get things you can’t buy here, like melatonin (for sleep hacking) and DHEA.
Give it a try.