Exposed Film for Flash trigger

Hi,

what is an alternative for covering the flash to use as a trigger?

Film days are over, so I don't know where one can get exposed 35mm film strip.

IR film is too expensive.

Rosco has some ND gels but I think that will just reduce the flash power not block the visibile light.

need it to be thin film type to wrap around my sony hvl-f20am.

any ideas?