SaraM777 - good advice.
I am unsure of your age. I would say if college age, go to the store and just go for it. If anyone asks, just say it is a gift, or for an aniversary or something. If you go to a place say like Victoria's Secret....usually the woman there are really there to assist (and make a sale, of course!). I would just walk up and say something along the lines of "I am very out of my element here. I am shopping for someone, they wear a size (insert size), and I am looking for something (insert whatever you are looking for....lacey, girlie), could you please point me in the right direction?" If they do anything that bothers you, just simply say, thank you for your help, I will try another store. and just leave. I honestly would suggest going to more a botiquey type store than say wal-mart because stores like VS, Fredrick's (although pricey), are specialized to that, and I feel like the "gift" type excuse would be more realistic in a place like that.
Or as far as thrift store type place is concerned. I am sure no one will say anything to you. Even if they did, you could always just snap back with a "what, have you not ever had to shop for a family member who is unable to before??" and I'm sure they will feel like a jerk.
I know it is not quite the same thing, as we ftm's just get mistaken for "dykes" but I feel you. I often always think people are looking at me when I am standing there trying to figure out what boxer briefs I would like to try. Most of the time, people generally do not care. If they do seem to care, just keep in mind you will probably never ever see them again.