well i usually talk about music for a few min. and then ask "you smoke?" and if they say something along the lines of "yes" or "smoke what?" then you can ask if they know someone you can buy from. and sometimes if im stranded i can walk up to ppl and just ask them straight up. works best if they look like they smoke
"Yeah, I just got in trouble for some weed." or "Just got off probation for weed." Something like that usually helped me.