I'd approach this by taking the mechanics of existing magic items and changing their flavor to match your goal.
You might have a hard time finding items that explicitly grant flight, especially at low levels but consider other movement bonuses. Any item that gives a bonus to movement speed, falling, or jumping/athletics could be flavored as your developing wings offering some benefits without being fully functional yet.
For example, as a heroic tier character you might define your wings as a level 3 Safewing Amulet (reduces falling damage) or a level 5 Cape of the Mountebank (a teleport reaction you could attribute to a brief burst from your wings). As you progress in level you might upgrade this item normally or define a progression path which swaps it out for the mechanical effects of other items. Perhaps at level 12 they become Battlestrider Greaves and at level 20 you replace them with a Ring of Flight.
Counting your wings as an existing magical item can also give your DM an easy way to keep the bonuses your "wings" offer balanced with the rest of the party.