If you want to buy single stamps, then the best place would be the USPS Self-Service Kiosk. The good thing is that they are open 24/7 and therefore, you can access them any time you want. You can locate these Kiosks outside postal offices. They are just like ATMs. You buy the stamps, you make the payment using your debit or credit card. An alternative option would be Winco grocery stores. They are known to sell stamps in both single and booklet forms.