How/where to buy bricks

Hiya, new to building outside of LEGO’s own off the shelf sets. I’ve found a moc on rebrickable that I really wanna build but it’s 1800 ish pieces. I have most of them but they’re all in sets I don’t want to destroy. Any advice on how to buy that many specific pieces all at once or do you have to just go to whichever store has them separately?
Thanks in advance :)