LCSC is typically the cheapest vendor for buying reels. Note, however, that the shipping fee is rather expensive (around 35€ to Germany), so it will only be worth the saving if you order larger quantities of components.
10nF0603 50V: 4000pcs of Yageo CC0603KRX7R9BB103 for5,20€100nF0603 50V: 4000pcs of CCTC TCC0603X7R104K500CT for4,80€