Weed (THC) is illegal in the Czech Republic, thus it is not openly advertised. Search for a "weed bar prague" in ordinary google search; various forums and reviews will show up. Then put some bar name into google maps and there will probably be a "user guide" in reviews. If not, then move to the next one - there are dozens, if not hundreds. If there is no guide and you're certain the bar sells weed, simply ask for it directly, usually the only requirement is to buy a drink first.