I wonder if the metal fillings left behind by a cut off saw would work ?
Probably depends on the sparks you want, and the teeth sizing on your saw, but it works, i use it all the time. I've also used turnings from lathe's, and mills. If your priority is pyro, and not production times, you can set up your cut and feed speeds so that the later two produces ready to use material. Doing it like this leaves you with a hot, clean product, without contamination from cutting fluids, since you haven't used any.
If your just grabbing whats "left over" after metal working, more often then not it ends up being soaked in cutting fluids. I haven't bothered to actually wash it. I might rinse it of, just to get the worst of it out, it seams to speed up the drying process, and doesn't leave a sticky mess behind. But that might depend on your cutting fluid. We use water-soluble synthetics. Supposedly biodegradable, and "green", yet it has to be collected by a waste disposal service, and is treated as hazardous material. (Only the thrown out fluids, the metal goes straight to recycling, if i don't steal it)