TBH I seek out bbfs whenever I can. All you really gotta do is ask. Some will ask for a tip and some will just slip it right in. Be casual about it so if she refuses it wont ruin the mood. Sometimes you don't get it and thats fine too. I've had it in low end and high end shops including (ginza etc).
It can be annoying playing the lottery when you dont have info beforehand though.