(our recommendation): The easiest way to buy your Ugandan Tourist Visa is on arrival in Uganda at the point of Entry of the Entebbe International Airport. You must bring a new and crisp 50 USD bill to pay for your Visa.
Apply online: https://visas.immigration.go.ug
Please refer to this document for step-by-step directions to apply online. After submitting your application, you will receive an email immediately stating that your application has been received. This email will also provide instructions for making an online payment of $50 USD. Within 72 hours you will receive another email confirming receipt of visa payment. You will also receive a separate email confirming that your visa application has been approved. Print the attachment and present this in Uganda upon arrival with your Yellow Fever vaccination card.