Sounds wonderful! I hope you find it and we can source it here. I like DE but the problem there is as soon as it becomes wet it no longer works. Well, maybe it does on slugs, never tried that.
I do sympathize with this challenge, having lived and gardened in the SF Bay Area and in the PNW also. The only thing I found at all effective was to mount twice or thrice daily patrols, hand pick and dispatch the critters. That did work to a sufficient extent they didn't destroy everything, but there was still damage.
I learned these slugs were an invasive import and not the natural fauna of the areas -- I forget the details but they came from Asia somewhere and outcompeted the locals (such as the banana slug which has a viable role in the ecosystem) and did that invasive thing we know and love so well --