Over at DAZ, you can find some Degas-inspired ballerina outfits by MAB and Will Dupré, for Victoria 3, Aiko 3, and Stephanie Petite.  Cris Palomino and Lucie sell poses.

Over at RuntimeDNA, Capsces and Lady Littlefox have a ballet outfit for Aiko 3.  sarsa and secretheart sell three texture sets (the "Land of Sweets" series) for the outfit.

Here at Renderosity, Cris Palomino and canary3d offer lace-up ballet boots for V3 and Stephanie Petite.  wenke has a massive pose set for V3, Aiko 3, and Stephanie Petite (with bonus poses for the ballet clothes from DAZ and RDNA).  secretheart has a fourth volume in the "Land of Sweets" series.  PickersAngel has a clothing set for preschool Maddie.  TwoSisters has a clothing set and expansion for V3.