You can find them there. I'd only use them when you need to...if you don't have a problem, don't fix it. The best prevention for external parasites is establishing a healthy environment for the chickens. You can dust the roosts, nests and the dusting spots whenever you like...I'd not dust the chickens with it unless you have to.
Is it ok to just use the sulfur or pyrethrin dust or both ? And how often should one dust the roosts, etc ?