You cannot buy medical marijuana legally because it is a drug that is usually deemed illegal. You may even lose your senses for a particular time if taken out of proportion. Hence, it is not available in the shops openly.
You can buy marijuana in medicinal form for patients of HIV, cancer and other intensely harmful diseases. Consumption of marijuana helps reduce the side effects experienced due to the treatments of these diseases. However, if you want to use marijuana for medical treatments, you have to receive authorization. This authorization is provided in some parts of the world looking at the conditions of the needy patients. The patients need to get this legal authorization in order to purchase medical marijuana from chemists legally permitted to sell marijuana.
They can avail you with the drug on presenting the authorization. It is a government authorization, which you need to verify. It is verified once you visit the online site and submit the needed details to them. Your information is then processed further, in order to provide you with membership verification that gives you the authority to buy or order your needed medicine. One thing you should keep in mind is to register only with a reputed online medical store.