How do I buy medical cannabis in Washington DC?

It is only legal to purchase cannabis in DC if you have a medical cannabis card.  
There are currently 7 licensed dispensaries in DC.
The allowed product types include flower, concentrates, edibles, topics and cartridges. Sale of transdermal products, seeds and seedling is not legal.

The licensed dispensaries are:

National Holistic Healing Center

National Holistic Healing Center
1718 Connecticut Ave. T300 
Washington, DC 20009
(202) 618-5635

Takoma Wellness

Takoma Wellness
6925 Blair Road NW
Washington, DC 20012
(202) 465-4260

Capital City Care

Capital City
1115 U Street NW
Washington, DC 20009
(202) 292-5071

Herbal Alternatives

Herbal Alternatives  
1710 Rhode Island Avenue, NW 
Washington, DC 20036
(202) 618-5635

Kinfolk Dispensary

KINFOLK Dispensary
433 Massachusetts Avenue NW
Washington, DC 20001
(202) 758-3795

Cannabliss

Cannabliss
4721 Sheriff Rd NE
Washington, DC 20019
(202) 388-3000

Anacostia Organics

2022 Martin Luther King Jr Ave. SE
Washington, DC 20020
(202) 845-8574