I you want to buy marijuana in Prague legally, you can buy CBD marijuana. Yesterday I discovered near to Charles Bridge little shop full of marijuana.
Difference between CBD and THC you can see in this video.
They have really lovely models.
I and my friend was really surprised by this discover. We never thought that something like this is possible in this country. But obviously is. I can only recommend if you like marijuana. In Prague you can really easy buy weed but you must know where. Usually it is some bar and you need only know how to ask. :) Best way is to ask some locals.
Most likely, within 500 meters, there will be some and no matcher in witch part of Prague you asked.
But best way to enjoy marijuana is go to the country side. In all village you can fine smoker with own outdoor marijuana.