@ronnz98 made the build for the Pixel 8a, and their install advice is to follow the installation method for LineageOS, but when using adb sideload to of course flash the iodé .zip. So in short, I think the boot.img you are looking for can be downloaded from lineage. So use the link to Lineage from column 1 in the table at the top for the instructions and links to most things, then use the link to the 6.2 build for the actual build image.
In the future I am going to take on builds for the Pixel 8a (and 9 series, and older 2 series, etc). I create the relevant boot,img, etc.