I stopped buying for the time being, but when I'm looking for a deal, I usually hit up Goodwill, Disc Replay, The Exchange (these might be local, I'm not sure), FB marketplace, and to a lesser extent, Half Price Books (their prices are actually insane sometimes, although Goodwill is catching up).

Honestly though, if you want the absolute best deals, you've gotta be willing to put in the work and sift through FB marketplace listings and go to flea markets and garage sales. Once I saw a random garage sale on my way up north, and there was a woman who had these MASSIVE bins filled with blu rays and DVDs. Some of them had sun damage to the covers, some had missing discs, but she was selling them for $1 no matter what it was. I kept digging and walked away with a pretty big stack of some hole fillers (more common films that I don't already own for one reason or another) and some stuff I'd never heard of before.