Experienced book sellers: Where do you source your best book pallets?

I’m getting more serious about selling used books on Amazon FBA and I’m looking for a reliable source for books by the pallet/Gaylord.
I’m not looking for a specific company recommendation just because they advertise online. I’m looking for advice from people who have actually been doing this for years.
If you’ve bought pallets or truckloads of used books before:
Where have you found the best quality pallets?
Who has given you the best mix of profitable books?
Are there any suppliers you would actually recommend from personal experience?
Are there certain libraries, charities, liquidation companies, book recyclers, or wholesalers that sell good unsorted inventory?
What suppliers should I avoid because they cherry-pick the valuable books before selling the pallets?
If you were starting over today, where would you buy your first pallet?
I’m in California, but I’m willing to buy from anywhere in the U.S. if the inventory is good enough to justify freight.
I’m especially interested in unsorted/untouched books where the valuable books haven’t already been pulled or scanned for Amazon/eBay.
I’d really appreciate recommendations from people who have actually bought and processed these pallets. I’m trying to avoid wasting money on a pallet of 95% junk.
Thanks!