Q. Where can I get a coil that works with your box?  

My controller boxes are compatible with 120V/100W coil heaters like d-nail.com's and these from China

Q. How fast do the coils from China ship? 

Fast! I got mine in 5 days.

Q. What nail do you recommend? 

I like Highly Educated's titanium e-nail parts. 

*HE Enail Insert

*HE InfiniTi Enail Adapter

Q. How sturdy are your units?

VERY sturdy. I parked my truck on my personal unit. Not a scratch on it!