I started wearing skirts about six or seven years ago, which is when I first figured out what slips were for. Because yeah, what are you supposed to do with an unlined skirt and tights? You can't walk three steps without it creeping halfway up your thighs.
Anyway, I six years ago I headed to the Bay (major Canadian department store) to pick up a plain black slip, and it was no problem. I only really needed the one slip for several years. But then about a year or two ago, when I started wearing midi skirts, I needed another, longer one, and also I could have used a full slip for this cotton dress I had. I was astonished to find that shapewear was all anybody really stocked. I don't do shape wear. I was eventually able to find a plain pencil-skirt shaped slip from Calvin Klein, but totally had no luck with a full slip.
I asked the gals at this local boutique I go to -- one that sells a lot of cute but unlined skirts -- where they get their slips. I know they wear them because both of them sometimes sport the intentional-lacy-hem-hanging-down prairie look (you know, when you wear a slip with a wide lacy border and let it poke out an inch or two below your skirt hem). Turns out all of their slips are from vintage shops.
I went looking for a slip again a couple of months ago, and was pleased to see, in the department store at least, that there seemed to be more selection. They also had pettipants, which intrigue me. Might give those a try one day.