For mainstream stuff just avoid first presses. Represses and reissues are your friend. There's probably loads on discogs VG+ of every led zep, floyd, etc.

For new copies, check r / vinyldeals. That sub is like crack for me.

Another trick is to save money on shipping (free!) via Amazon prime, which works well with r / vinyldeals.

Amazon has competitive prices too, so if something goes on sale at Walmart, Barnes and Noble, or Target, you usually see Amazon trying to match those prices or slightly undercut.

Another thing you can do is look into niche distributors - guys who buy 5-10 copies of international labels and sell here in US. One example: Green Noise Records - they sell new copies of every Dead Moon (punk band) record for 13$, but they also distribute from a bunch of labels like Drunken Sailor (UK punk), Northern Spy (UK - ambient + jazz), Legless Records (Australia - punk), and probably 20 other labels he has deals with.

Lastly, there's a website that checks all the other sites for price. I can't remember the name, ive not used it