These look different but have a ribbed sole.

This one also has a ribbed sole.

Here's a third pattern with a ribbed sole.

You may be able to find a pattern on Ravelry.com.

I got all excited for you when I saw these slippers – "Booties for
Grown-Ups": http://www.ravelry.com/patterns/library/booties-for-grown-ups

But soon realized that garter stitch is not ribbing ... but it could be
changed to ribbing.

The soles of the Jasmine Slippers could also be changed to ribbing from garter
stitch. Found here: https://www.youtube.com/watch?v=EyW7s2ndmxo

The soles of the Knitted Moccasin Slippers – here: http://www.crochetandknitting.com/mocslipk.htm –
could also be changed to ribbing.

When I first saw your slippers, I said that they looked like Pocketbook
Slippers ... two-toned Pocketbook Slippers.

Seamless Salomas Slippers by Megan Williams are a type
of pocketbook slippers and they are the nearest to what
you have (which might not be all that believable, coming from someone who could
not tell the difference between garter stitch and ribbing). lol
Seamless Salomas Slippers can be found here: http://www.ravelry.com/patterns/library/seamless-salomas-slippers

You might try Searching for various other Pocketbook Slipper
patterns.