Quorn Foods is the premier producer of meat alternative products worldwide. Based in the UK, they have been operating in the US for more than a decade. Quorn's products are made from what they identify as "a naturally occurring protein in the fungi family," which they call "mycoprotein." The protein is harvested in a fermentation process similar to brewing beer or making yogurt.
Because Quorn products are soy-free, they're naturally non-GMO. They also tend to taste very similar to the products for which they substitute. For example, the first time we tried the chicken nuggets, I had to take the box out of the trash to make sure they didn't actually include any parts of a chicken.
Even if you aren't a vegetarian, Quorn has about half the calories and fat of meat and is very high in fiber and protein, so it's a good alternative to try. You can find Quorn at most grocery stores or national chains like Target and Whole Foods. Availability of specific products varies.