If you are self-catering, Kinloch Rannoch has a good general store with off licence called The Country Store which is well stocked with most things you might need.
The Tummel Valley Holiday Park in Tummel Bridge has a convenience store with off licence.
Or you could stock up with picnic food at The Country Store or the Riverbank Cafe and take off into the wild. Why not climb Schiehallion and picnic at the summit.
But please leave no trace of your visit behind and only take photos and memories away with you.