LightLeakLove
I'm not sure if these qualify as what I understand a light leak to be, more like off camera flashes of light creeping into the lens, but you can't beat reality and these are real. The free samples can be downloaded if you sign up to their newsletter, a pretty low risk affair considering what you get, which is a 1280 + 720 clip of assorted light flickerings from their range.
HolyGrain
They sell all types of film grain, 35 mm, 16 mm and even 8mm and at reasonable prices. They also provide a nice free sample of 35mm grain with a just the right amount of added dirt to look authentic and not stupid. This freebie also comes at 1920 + 1080 and you don't have to sign up to anything.
Vegasaur.com
The motherload. 20 free samples of Light leak flashes at a useful 1920 by 1080 aspect, Five free samples of Particulars at 1280 + 720, these being filmed dust float elements, and one 35 mm film grain (no dirt) also at the smaller 1280 +720 size.
And since we're talking dust elements, not that he needs the exposure, you will find a swag of nice high quality filmed powder over at Video Co-Pilot
And it doesn't end there, there is free video and elements to be had over at Videvo
