I have used Tar Heel State Firearms for mine. They did 5 receivers for me. I had to look around for awhile because my CZ is a polymer and I couldn't find someone to do it. I couldn't be happier with their work. I have one of those insanely long trust names. They were able to engrave 3 lines of text at 1.6mm in the trigger area without disassembling. Looks better than the factory engravings. Best thing is they only charge $30, let me send all at once so I saved on shipping and I had them back in 3 days. They are my go to engravers from now on. Super friendly and helpful and being that they are a Class 3 dealer they do all of the NFA stuff. Actually stopped me from making a mistake myself!
This is where I found them.
http://www.ar15.com/forums/t_6_20/442659_Tar_Heel_State_Firearms___F1_suppressor_engraving_A_.html&page=1#i4286538