Is It Possible to Obtain a Non-Immigrant B Visa Within Thailand?

The reason not that many people do it IS the documentation required for an in country Non-B is far more stringent than what is required at a thai consulate to get the Non-B. Many employers can't meet the in country requirements to get Non-B's issued because of that.

Yep, totally possible to do a 90 day Non-B visa inside the country at most immigration offices. You need to be on a 30 day visa exempt, 60 day tourist visa or a 30 day extension from one of those, you need between 15 and 21 days left on one of those stamps AND you need the documentation from your employer.

You apply pay the 2000 baht, the application goes "under consideration between 14 and 20 days (that's why you need REAL days left on your stamp) and then you go back and get the Non-B visa and a new 90 day stamp inked in to your passport.