To purchase DNP in the United States requires certain types of licenses (you have to prove you'r edoing research or have some other need for it) that are not easy to come by.
I suspect that most people selling capped DNP have found a source, buy it (it's cheap as chips) and then resell it at a massive upscale. So long as they are not idiots about using a scale, it will be safe. You have to mess up pretty badly to mis dose like that. That's on top of the biggest pill being about 1 gram anyhow.
I'm sure there are hombrew recipes to make it. Anybody with the organic chemsitry background to run that synthesis won't be an idiot about the scale or mis-measure that badly.
The biggest issue might be contaminants I guess. Then again, if you knew about all the bug parts in your food...
Again, find a TRUSTED long-term source where people have used it without problems and there should be minimal risk in terms of dosing per cap per se.