If you have difficulty mounting filter, use a little hot glue, silicone sealant or rubber cement to hold it on.I`ve found this one 5-pack 1x Polarizer Filter 4x Neutral Density filters for DJI Phantom 3 Standard
It`s from Europe I guess, but fitting filters is very unreliable.
I saw in the description that it comes with "Double Sided adhesive pads for installation", which should hold it on pretty well.