you could put spawn into a clean straw bale after giving it a good soak !!! You can also use sawdust bales , as long as they are of untreated wood . They work perfectly OK.
There will always be some fresh branches that have been chopped off somewhere , phone a local tree surgeon , they will have tons of it. The reason you need fresh wood is it may get infected with other types of fungi if it is old stuff ...MAY !!! I used an old log that had been cut down at least 2 years , just to see what happened ....it worked fine . Grew perfect Shittake mushies.
You can also cut plugs from your stock logs once the fungi has spread throughout the log , and use those plugs to inoculate new logs .....thats how they get the ones you buy !!!
cheers
Russ