There's plenty of online resource, but studioscale.com only holds a small portion of main offenders.
Best advice, be resourceful, even looking at builds, or built kit pics can turn parts up, and there is nothing like having in hand kits to look at too, even then parts get missed, its a big adventure, but a buzz nailing parts
Here is one ive found a few parts, not many on here, but still worth having, i bagged a very useful Mak donor on here for Jason a long time ago, so it has its uses.
Modellbau Kit-Ecke - Modellversium
lee