As an organizer I have found receipts in the funniest places. I'm still not sure why some people keep receipts if they don't have a secure spot for them so they can find them later.
You really have to make a conscious effort to keep them together in one place until you get home. So you might start with keeping them in your car, purse, or pocket and then transfer them to a secure spot once you arrive home.
Read more about what to do with the receipts you've collected: