I always prefer to use fresh hardwood chips straight out of my old chipper machine from
trees and branches I cut in late winter/early spring. Alder, oak, poplar,
apple and cherry seem to work the best, King Stropharia/Garden Giant will also grow on Douglas-fir chips and sawdust as well. Doug-fir sawdust and chips would probably be easier to find for free than hardwood chips if you live in Idaho. But when my chipper breaks down (which happens frequently), I sometimes use alder chips (for smoking salmon) that I buy in bags at the store combined with hardwood (usually alder or maple) sawdust from a local cabinet and ukulele maker. I usually use a 50-50 ratio of chips to sawdust. And if you use dry chips or sawdust from the store, be sure to soak the chips/sawdust for at least 24 hours in a
bucket or large plastic bag before inoculation. Keep in mind that it's usually cheaper to ask tree service companies, furniture makers and even chainsaw sculptors for free
wood chips and sawdust than to buy bags of chips or pellets at the store. Good luck!