Try breweries also. The larger ones receive chemicals in them. I got mine when I worked at Upland a few months back.
In what part of the State are you? There is a guy in Brown County that sells them and I could find his phone number.
Gurman Container in Terre Haute has them.
?
A 5 gal bucket filled with toybox sand should do the trick.
The Box O' Truth #7 - The Sands O' Truth - Page 4
Would probably be better to just make sure there is no round chambered either way. This seems like an excuse for not being safe enough.
I understand the concept, but if you aren't either 100% sure it'll go click or bang, check again or put the tool down.