>>7560289Buy the pieces directly from LEGO, if it's not on Pick-a-Brick you can buy in the replacement pieces on Support.
You can pretty much buy anything that's not licensed. So you can't buy, let's say, that Overwatch girl, or pieces with its symbols, but you can buy any other piece, if it's still in production.
The prices are pretty good, almost Bricklink level.
I'll do it with the Overwatch pink pieces.
Also, I read on LEGO that you can download the old Instruction Books from fan sites, or the new ones as PDF.
So it's considered, by LEGO itself, that it's okay to download the old instructions bookles scans. And it's okay to use the official ones, even if you don't own that set.