"Why aren't there private distributors of human bones in the US?"
Technically, there are! However, when people donate their bodies to science (cadaver labs, body farms, skeletal collections, museums, etc.) They usually remain there forever. On a rare occasion, an institution could close and have to find new homes for its specimens however usually contracts with these facilities promise the remains to stay there or are cremated.
“Can I visit these collections?”
Usually, the answer is no. Most collections are not open to the public due to privacy and respect reasons. However, there are some museums that have skeletal remains on display from consenting donors or donors who no longer have living family members.
“Why did medical skeleton companies such as Kilgore, Clay Addams, and others stop selling skeletons?”
This is a long and complicated topic, but to keep things short and to the point, human skeletons are not allowed to be donated for sale because it often takes advantage of people. It is important to current doctors, scientists, and institutions to be ethical. They want to respect the individual who donated their body and their family.