Medical food and meal replacement shakes existed well before soylent, they're usually intended for people who are on a feeding tube. There's a lot of common brands like Ensure, and you can find probably at least one decent option in a reasonably well-stocked grocery store. If you want to order online there's some interesting stuff like: http://www.functionalformularies.com/liquid-hope-information...

Soylent's main distinguishing features are marketing and sloppiness. They are also a little cheaper than other options, but here as with most things, you get what you pay for.