http://uwb25d43nnzerbozmtviwn7unn7ku226tpsjyhy5n4st5cf3d4mtflqd.onion/posts/grapheneos/index.html
It is available on their GitHub Releases page — app-arm64-v8a-release.apk of the latest release is what you want (arm64-v8a is the processor architecture). If you need apps that aren't available in the Play Store or Accrescent, install Obtainium in the Owner user profile (and don't disable it), after first verifying its authenticity with AppVerifier (which is available through Accrescent).