I just added "Mass healing word" to my Bard's spell list from Tasha's (p. 27), but I can't find if there is an official spell card release for it. I've got the decks for Bards, Rangers, and Xanathar's, and I can't find it. Anyone know if this card is available?

The spell Mass Healing Word is not a spell on the Bard, Ranger or Xanathat's Guide to Everything it's on the Cleric list in the Player's Handbook.

Thanks! 

So I can get the spell card from the Cleric deck.

But is there a way for me to add it to my sheet in DNDBeyond as a Bard?

If you own Tasha's, or your bard is in a campaign where content sharing is turned on and someone own Tasha's, then you want to go into editing mode in the character sheet, in the 'home' tab turn on/red/right 'optional class features,' and on the class tab select the 'optional feature manager' and turn on (check box) the Additional Bard Spells feature.

If you don't own Tasha's, since Mass Healing Word is a spell from the Basic Rules, you can use the homebrew tools to create a copy of it (create new homebrew spell, select the spell to use as a template) and add Bards to the 'available for class' section.

Thanks!