From Ma’at (AL admin):

For true polymorph, shapechange, wild shape, and find familiar players use the spells' and ability's rules. Outside of the lists of beasts provided (wild shape and find familiar), any other stat blocks allowed by the rules are at DM discretion, not ALPG or ALDMG guidance.

https://discord.com/channels/516367331358801950/1049794647322546246/1361449879460778114

ALPG v2026.2 (page 4):

DM MATERIAL USE

If your character’s abilities (e.g. Druid Wild Shape, Polymorph) need DM information, those limited details come from the current Monster Manual or, at your DM’s discretion, other sources from your campaign.