I've been using this set for about 3 months and I'm quite pleased with them.

3 Piece Filter Kit (ND16-ND32-ND64) for DJI Phantom 3 4K,DJI Phantom 3 Standard, DJI Phantom 3 Advanced, DJI Phantom 3 Pro, DJI Phantom 4 Quad-copters https://www.amazon.com/dp/B01DTK5KBQ/ref=cm_sw_r_cp_apa_EkpHzb69W9KE4

Sorry for the long link, I'm on mobile.

Edit: ignore the Pro, Advanced, blah blah blah on the listing, these are for the 3 Standard and only the 3 Standard.

There's this set Can't speak to the quality though.