Harking back to my childhood days eons ago, fireflies don't 'keep' well at all. I used to catch them and put them into a jar with some green leaves in it and holes punched into the lid and if I was really lucky, they'd still be alive in the morning.
Aside from the fact that fireflies are quickly disappearing as a species due to encroachment of man resulting in loss of habitat, the other issue is they are not indigenous to the whole world...only some areas. Add to that, they are only actively 'blinking' for a few hours shortly after sunset...and that's usually in calm, semi-humid to humid conditions. So unless your wedding is properly timed, releasing them and having them blinking won't happen.
Perhaps the biggest issue, however, is finding enough fireflies to be in any way 'spectacular' upon release. And that's assuming they all start blinking (or keep on blinking) after their release. In short, do you know of any location, perhaps within 5-10 miles of where the wedding will be, that has perhaps 200-300 fireflies easily captureable in a short time? And how many people would be needed to do this? And securing permission from the land owner? The land owner may want to 'keep' his fireflies as every one caught this year is maybe 20 fewer next year! And no, I am not aware of anyone or any organization that raises fireflies.
While the idea of releasing fireflies seems very beautiful, I think it would be more effective if the wedding were held where the fireflies naturally exist. Unfortunately, that is usually in rural settings with little lighting as the bright lights of 'town' or even the lights of passing automobile traffic reduces their numbers (and blinks) significantly.