I haven't seen plastic pants in a pharmacy, but I can tell you that from my experience buying diapers at a pharmacy, and I do it often, is that nobody cares what you are buying. I have several pharmacies near me, but I generally go to one that is in an old fairly abandoned strip mall that not a lot of people go to anymore.
Usually, I'll just scan the parking lot real quick to see if there are any cars of people I know, so far it hasn't happened and buying the diapers has been a breeze. The cashiers are always professional and I just act like its nothing, no different than maybe buying aspirin or dishwashing detergent or something. Interesting side note, a few weeks ago I went in to buy a pack of depends and some other things I needed, and there was an older lady in front of me buying a weather radio that has a hand crank so it will work when the batteries are dead and the power is out. She took it out of the package and the cashier was showing it how it works. Anyway, the cashier saw me behind the lady and asked the lady if she minded if she ring me up. The lady of course said yes. I asked about what that radio was and they told me and we kind of struck up a conversation. Neither of them batted an eyelash at the depends at all. So yeah, it's easy, you shouldn't be scared of buying them.
One tip to be discreet, don't buy any items along with your diapers that could be age restricted and would require you to show ID. The obvious ones are cigarettes and beer, but some locations or even some stores have policies that age-restrict other items. I've been ID'd in the past for spray paint, pocket knives and cough medicine, all of which I did not intend to use illegally, just thought I'd mention not to buy any of those with your diapers to be on the safe side.
