Frequently asked questions How do I buy medical cannabis in Montana? If you have a medical marijuana card or certificate letter, you can buy cannabis in Montana at a licensed dispensary or use a licensed delivery service. Also, you can grow your own cannabis if you have not chosen a dispensary – up to 4 mature plants, 4 seedlings and the amount of usable marijuana allowed by the department by rule (1 oz. usable cannabis). If there are two or more people who qualify for a medical marijuana card in the same residence, the maximum is 8 plants, 8 seedlings and 2 ounces of usable cannabis. You must keep any usable cannabis over the 1/2 ounce weight limit in a private space, out-of-sight, and locked away.