Most parents issue their own diplomas or their children recieve them through the curriculum program they have chosen.
Some states view these in the same light as a diploma from a public school, a private school and some a GED. There are a few states still in the dark ages that don't aknowledge homeschool diplomas.
Check your local college and see what they require for homeschoolers. Most now have that information on their sites or available if you call.