alt Hacker NewsCarrier-sold Pixels generally don't have "OEM-unlockable" bootloaders.
Your best bet for now is to buy a new Pixel direct from Google, or a used one from eBay that the seller advertises as already having GrapheneOS on it (or otherwise guarantees that the bootloader is unlockable). These ones are worth a lot more than the ones that can only run Google/carrier Android.
https://grapheneos.org/install/web#prerequisites
I own two GrapheneOS Pixel 7 units, which should get any Google blob security updates (which GrapheneOS incorporates) through October 2027, and GrapheneOS may still support it with source updates after that. So in a year or so, I might get the GrapheneOS Motorola if it's available, or a later Pixel. (I never buy these new, since I don't want to carry a several hundred dollar phone when a 2 gen old one is still great, thanks to GrapheneOS.)