If you to the first link I posted below, scroll down that page a little and it gives you an option to put your zip code in and it will tell you the stores that carry it near you:
(Just a note, they now call it high protein but it's still low carb, look at the picture on the website I just posted. They must of just updated the label. The supermarket near me carries the older labels that say low carb.)
This link happens to be for the chocolate, but there is also vanilla. I can tell you also that walmart has low carb slim fast you can order online.
Here is low carb vanilla at walmart.com, there is chocolate also, and they have package deals,you have to search for "low carb slim fast" on their site:
The above is for the ready made drinks prepackaged.
There is a high protein powder form, though the powder is considered low carb per their site, it has way more carbs than the premixed ones so I don't recommend that one. I only recommend the premixed ones.
Hope that helps!!