You need to "just do it".
Assuming that you are presenting as a man . . . If you are looking a nighty or perhaps even panties, you might get away with telling the SA they are for your SO. If you are looking for a bra, tell her it is for you and ask for help. Guys just don't purchase bras for women and she won't believe you anyway. It can be fun to get some help and if you are like me, you need the help.