I use:

www.morrisonbmw.ie
www.hubauer-shop.de

as my primary sources for OEM parts. I don't always automatically go for the cheapest parts, sometimes it is good to buy local (well Tipperary).

I'll also use Otto.ie too as they usually stock the top branded parts like Lemforder, NGK, Sachs, etc.

I just found over time that local, non affiliated motor factors stocked the lesser quality stuff. I was also a fan of Cotswold in the UK but the 33% hike in prices brought them to be more expensive than others now.