Where are these signed?

Usually on the bottom of the stone, below ground level.
Sometimes one can see the signature on the bottom of the stone -above- ground level.

Much of the time I get who did what from what published sources were able to figure out.

I may have mentioned this to you before, almost certainly. Horizon magazine, or possibly American Heritage, ran an article on these stones c. 1970. As a kid, I was blown away, as I am when I read your posts. What caught my eye was one of the examples had been defaced by adding nipples to a lady's breasts. This was left in the article to alert people to vandalism, it was no accident. In Plano, the oldest stone in the cemetery was 1858, as I recall. Limestone, because that is what most of north Texas is made of. About 6 miles north of my house was the site of the last Indian killing in Collin County, c. 1856 or so. A family had been killed by some tribe, probably roving Comanche.
I know there has been much scholarship on the older stones up north, and we are starting to catch up here. Much of my family is buried in "scraped earth" ground in Castor, La. Once a year the locals go out and scrape off the grass. I rarely see signatures on stones, modern or old. We have a schoolteacher here in Rockport who's stone has seven cartoon cats around the name and date. This is the most delightful gravestone I have ever seen.

http://cultured.com/images/image_fi.....l_of_grief.jpg

I want something like this, surrounded by twisty yew trees and on a barren hill, with benches so kids can get high and have sex there and enjoy the best Halloweens of their lives for the next thousand years until the stone is incorporated into the last ditch defenses against the unstoppable giant chitinous slavers from Alderberan VII.