Sure, mixing chemicals is a lot like learning to drive a car. Bad things can happen if you don’t know to turn the wheel when you come to a curve in the road. 
Toadspittle said:
Well, I’m not remotely interested in preserving vegetables. Actually, the truth is I want it for a magic trick I’m working on. But it’s such a good trick, I probably shouldn’t reveal the details on here. Sorry.
I’ve ordered it from the spice company I mentioned above but if that doesn’t work out I’ll try the other suggestions above made by duckster and curlychick.
If THEY don’t work out then it looks like I’m gonna be digging out the ol’ battery acid and foil idea. So using a metal pan to mix it in is a bad idea then?
What would I use - plastic? glass? wood? what?
For what it’s worth, curlychick, the chemists I asked assured me that the big chemical companies wouldn’t deal with me as a private individual. But you say different so what do I know? Anyway hopefully it won’t come to that if the Great American Spice Company don’t let me down.
Look at the link I provided earlier. It offers step by step instructions. You would need to use a glass container, and preferably have access to a fume hood. It’s not as if alum is a hard to find substance, however. I found all of these sources for alum in about three minutes.
Have you looked in the spice aisle of your local grocery store yet ? I picked up a ounce of alum there last year (Trend brand) for 99 cents. -Forgot about it til now, or I’d have mention it earlier. McCormick’s will probably charge $3.49 for the same amount, so do a little comparison shopping.