Where can I find them reasonably cheap?

This is a pistol, right?  Are there certain rules/regulations to building one (FFL required for reciever, short barrel tax stamp)?

Reasonably cheap?  Maybe EE.  I've seen them between $525 and up (yugo krinks).  Bulgy krinks go for big bucks now.  Good luck in your search.

Ok i'm not sure I understand your ? but i'll try to explain what I think your asking
Some of people make krinks into pistols but they are set up to be built into a SBR .
least amount to shell out for a krink parts kit will probably be $525 to $575 for a Yugo I was trying to sell a Bulgarian 5.45 a short time back for $600 and had a couple people said they would take it then never came with the $ would be a great SBR kind of a waste to make a gun like that into a pistol.  
If you make it into a SBR you will get to pay a $200 tax stamp for that gun
If you want a pistol you might look into a AMD-65, or DPH Arms had pistol kits not to long ago
And yes if you buy a complete rec then you will need to have it sent to your ffl.
hope that clears things up