You can find those exact pots at mouser.com for $1.44 and $1.46 each in SINGLE quantities. They would have the linear taper and 100K and 500K values needed. The only feature that I would prefer would be to have CENTER DETENT so that the middle position would be visually easy to see. Now you have a GRAPHIC EQUALIZER!
I have shied away from linear pots because I don’t have an easy way to cut long rectangle holes in aluminum enclosures. If you have a secret, let me know.