If you have a fleet farm near you: https://www.fleetfarm.com/detail/american-excelsior-company-7-cu-ft-premium-horse-bedding/0000000228850 This is a great option. I know some tractor supplys will carry it, but it’s not listed on their website. You’ll need to call them to check.
You can also check out laboratory supply stores in their area. Many of them will carry Sani-chips or other types of aspen bedding.