[JayG]

Sulphur powder is alternatively known as "Flowers of sulphur" - plenty of suppliers if you Gurgle that. 

[Trillium]

That's what I figured, Jay. It's very common but not to every gardener's 'taste'. 

My sister buys hers by the 50 lb bag and dusts it over surface pests as well, particularly roses. It washes off after every rain but her roses are now in great shape so I'll have to start dusting mine as well because I get masses of those horrid green caterpillars that strip the leaves.