Advice about where to buy LEGO spare parts for MOC projects

Hi everyone! I’m very new to LEGO MOC and I’d like to ask your advice about where I could find/buy some LEGO spare parts to build MOC projects. Up to now I just downloaded some free-available instructions of projects made by some users on Rebrickable, but I’d like to make my own creations once I get more experience. So that’s why I’d like to have your advice. Where do you find spare parts for your creations? Which online shops are the best (and,if possible, cheapest)? I’d prefer European...