As far as I'm concerned, humiliation is part of the fun. First thing I do is see if there's a self checkout or at least an older lady at the register. Then I go to any men's clothes near the panties and scope out what I might like. Then I jump over quick and grab the right size. Maybe buy a magazine to hold in front of the panties while you're going to the register. Another think I do sometimes is go to Walgreens Pharmacy. The panties cost more, but it looks more like you're buying them for a girl that ran out on vacation or something which is a good excuse if you get caught.
Last option would be to buy them on Amazon or some place but different brands sizes fit differently.
Good luck.