Aye, it depends on what you have available locally. Not all brands are sold everywhere. Sweetlix is a great one, but hard to find unless you have someone who stocks it locally. I have to stock up on it when I got visit family in another state, since I can't get it here.
TSC has manna pro goat...
A hay higher in alfalfa is definitely great, and most do quite well on just that, plus mineral. So if the rest are great, she might've had something going on. Some goats are just more thrifty than others.
A mineral block can lead to mineral deficiencies. They are mostly composed of table salt...
A fecal test sent a vet is a simple way to determine if there is a worm load, how many, and what type. Given her body condition, worms are a strong possibility.
What type of hay are the rest eating? Not all hay is created equal. Not all hays have a balanced calcium:phosphorus ratio. Or a high...
Animals who are ridiculously full of worms often look bloated. A heavy worm load will rapidly cause a goat to lose body condition. The worms in the gut absorb nutrients that are consumed by the animal.
Plus, an animal who is starving can also develop ascites, a fluid buildup in the abdomen...
DO NOT bury or dispose of her yet. If you want to know what happened, you need to get her to a vet ASAP for a necropsy. That is the only way for a conclusive diagnosis at this point.