I found some information for you, and I will attach the links. As far as I can tell, it is legal to purchase mercury, but I have yet to find out how this can be done! I am attaching two links for you: One is about a guy who brought forty pounds of it to a bar, and the other one is a link that explains about government regulations and health info. If it was illegal to possess, the bar guy would have been prosecuted. He may have poisoned many people as he spilled half of it in the bar and then buried the rest in a kids' sand box!
From what I found, I think it is legal to own mercury in NE. I hope this helps you out!
Hopefully these links will help you in your research. If I can help further, please let me know! Good luck in your research!