for consumable parts like pumps, brakes etc, you'll be suprised how many parts can be bought at regular parts supplier, like this one
Just a moment...
WWW.AUTO-DOC.FR
Mecatechnic is good for all the MVP parts and other and cheaper than walloth
/img/imgNew/photos/opengraph/OG-meca.jpg
Pièces auto BMW E10
WWW.MECATECHNIC.COM
Joachim at BTS is great for new old stock and has remade a variety of pistons. If you don't see it you can email him in english
https://www.bts-autoteile.de/index.php/de/
Used parts are on Leboncoin, but you've probably found that already. Rare and expensive used parts can be had from Cahsel in Germany.
NickVyse's post in Europe based parts suppliers was marked as the answer