Bonding Runestone, Rank 6

The is an  runestone.

Low-rank 1-5 Bonding Runestone can be purchased within the Sage's Shop from Bradda the Sage.

Mid-rank 6-10 Bonding Runestone can be dropped from Lockboxes. (depends on lockbox release)

Zen Market Companions tend to come with Bondings. You may also get Bondings from most the Packs sold on the zen market. Companion Upgrade Bundle can give you Bondings.