fsverity-release.x509.der is part of AOSP and can be downloaded at https://android.googlesource.com/platform/build/+/d24991a26e94fa298bed8ff9d5f77b9e9cd4b7a7/target/product/security/fsverity-release.x509.der
play_store_fsi_cert.der and gms_fsverity_cert.der are distributed to Android's GMS partners. For internal reference, these files were added in ag/10445919 and ag/13998342.