Since you're new, you should know that etiquette on this forum frowns on posting the same question in multiple places. You should decide what the most appropriate forum is, and post only in that one.
Home distilling in the US is illegal, and forum rules forbid discussing it here.
I'm not sure what's special about barley that has been used to make whiskey, as it won't have any whiskey character to it at all. The whiskey character comes from the barrel aging after distillation. Spent grain from a brewer's mash should perform the same way.
If you have your heart set on barley that was used to make a whiskey wash, look for local craft distilleries. There are more of them all the time (even tho licensing is a PITA.) One of them would likely give or sell you some used barely.
Brew on