Dude, I totally relate to this. I started dressing mostly male a few months ago, and kind of get freaked out even thinking about wearing women's stuff. I've bagged up all my girl clothes to take to charity, except for a few collared shirts I still wear to work since I'm not out at work yet. Otherwise, it's all slacks and polos. I have one pair of women's shoes in a men's style, and one pair of women's shoes that are close to men's style, fruit of the loom briefs, and men's socks. I have 2 men's dress shirts I won't wear to work until I'm out. No ties yet.
If you're out to the person you're shopping with, just tell them you're looking for interview and office clothes. "I'm a guy, and I'm not wearing a dress to an interview. Or ever. I don't do drag. Now help me pick out a tie, ok?" In the meantime, don't shop with anyone you're not out to. You know what makes you dysphoric, so don't put yourself in that position again if you can help it.