Try using a pocket fan with nylon fabric or sticky tape on the wings. Bring it close to your fingers and record the flapping sound. Modulate al gusto. Find a fan with extra-quiet motor. You can also get the type with soft fabric type propellers – those are perfect. Post-process with pitch ‘n time.
– Frank Kruse
If you need source wing sounds, there’s a dedicated Insects and Swarms SFX library available.