A number of years ago I was able to find a great deal on expired canisters on Yahoo Auctions, but that's not an option any more...
Fields and producers down here in SoCal are worried about fires and normally don't allow smoke grenades, even the cold-burning kind.
I've been able to use them in the past with one small change: water. The orange marine smoke grenades are made to float and work in water, so I'd hack a few 2-liter bottles in half ahead of time, fill the bottom half with a few inches of water on the field, drop the grenade in and let 'er rip!
It's a win-win-win: The producers and field owners feel safer knowing the "pyrotechnics" are surrounded by water, their insurance companies are happier and don't raise rates and YOU get to use these schweet smoke grenades!
Of course, always check with the event producer before using anything on field that is out of the ordinary and clean up after yourself.