I can’t comment in the specific of this case, but in general adult LEGO builders will acquire pieces through a range of methods - the biggest being bricklink -

I can’t comment in the specific of this case, but in general adult LEGO builders will acquire pieces through a range of methods - the biggest being bricklink - a website where independent sellers sell individual pieces, often which they’ve ‘parted out’ retail sets to acquire. LEGO has a similar service (‘pick-a-brick’), or in LEGO stores there is a ‘brick wall’ where you can pick up a small range of individual pieces (sold by volume - i.e. you fill up a box of pieces) and if you ever find yourself near Ulm, LEGO-land Deutschland has a physical location where you can buy pieces by mass. But in summary, I don’t think there is a magic bullet - it’s an expensive hobby!