Hi Tom,
IPSF is the only source that I know of. They seem to reproduce well in reef tanks, so you might be able to get a few from a fellow hobbyist.
I really do wish IPSF would correct their ID. Fine, call them 'strombus grazers', but they are not Strombus maculatus. (My photo gallery shares a couple images of this species.) These are not at all a Strombid but rather a Columbellid.
Cheers,
Bob