Where can I get a replacement set of boxie keys

My mechanic lost my original key set.

Is there any way to get a new set of boxie wagon keys with the GM logo on them or is that simply impossible?

When you say boxie, are you referring to the box generation of wagons ('77-'90)? If so, no it's not impossible.  Check eBay, they're all over.  You'll just need to know your year's key codes (e.g., A/B, C/D, J/K, etc).  Hope this helps.
http://www.ebay.com/itm/GM-J-K-Key-Blanks-Original-GM-Logo-Key-/320714231838?hash=item4aac0ed01e:m:m9YmvQy5EmOPt9hkK4OOBUQ&vxp=mtr

well, that was friggin easy.
Good on GM for doing literally the same thing for 35+ years such that ebay just stocks the key blanks. Now all I need to do is find a locksmith with one of those old-school key copiers instead of the newfangled automatic JUNK!

Just about any good locksmith can make accurate copies of your keys. It may cost a dollar or two extra but it's worth it.

My local Home Depot did mine.