I'm getting into custom builds now and I'm sick of constantly finding i dont have what i need. Usually because I need symmetry and only have one of a specific brick. I'm wondering how builders optimize their collection so that this happens less often.

Like what's a good way to bulk up my brick types so that i can get through a build without constant roadblocks? Or if you were to start all over, what would you buy to have a versatile collection of bricks that allow you to build whatever comes to mind?

And secondly, obviously you can't have it all, there's always going to be a specific thing in a certain color that you dont have. So what are the things not to focus on when bulking up the collection? What are the mistakes you learned from?

Lastly, do you use non-lego bricks?

If you live near a Lego store, you can visit their pick a brick wall and stock up on parts that are available at the time. The stock at mine doesn't change much, so I only visit about once per year.

Have you tried Bricklink? Plenty of sellers on there that will have the exact parts you need to finish projects or stock up.