Many sources online. Just go to google/yahoo and type in 'safety flare' and then modify it with '20 minute burntime' and '30 minute burntime' to get some sites.
Since it's combustible, there is a hazmat charge involved . It is 20 bucks, so it'd be in you best interest if you're going online to either order a large case for yourself, or go in with friends and split up a case so that you can also split up the hazmat.
Locally, you can go to auto supply to get them, and you can also go to boating supply to get them as well. Auto supply will get you the best chance at the larger ones, like 30 minute, while boat supply seems to focus on meteors and short duration handhelds. For automotive uses, I recommend keeping 20 minute flares. They are easier to store, and IME the 30's are often more than a private party might need.
I carry some in my cars, and it's amazing how durable those things are. All of that Florida summer heat and they still work when I rotate stocks. I don't mind carrying them in a car. The design of the things are such that they'd never ignite via impact. The flare ignitor is well protected, and if the thing got ignited by a heat source, that would generally mean that the car was on fire to begin with.
Downside is using them near a crash where there might be gas leaking, or placing them on dry grass and such, both cases where there might be a chance of fire. Thinking a little before using them will go a long way toward increasing safety.
Shame you don't know a cop. You could gank a couple from him.:naughty:
Just kidding.
HTH.