Looking to make this gun full auto . Do I need to buy a trigger assembly to make this full auto ? I live in a state where we can own full auto weapons with tax stamp . Any suggestions as to where I might find trigger group or lower half that would make this full auto

The NFA FS boards on subguns.com and sturmgewehr.com are good places to start.

HK transferables can be confusing. Not all will suit your needs thanks to the variety of approaches to conversion taken, ie., sear, registered receiver, married receiver, etc.:

HK conversions

YIKES , NO. You can't just buy a G3 trigger group and slap it in a 91; first of all they are different from a G3. and they won't fit anyway. You have to get one made for a 91, BUT,
FIRST thing you have to do is to find a registered rifle, or maybe trigger group, that already is registered and transferrable. Then a Class 3 dealer will tell you what to do from there; he will submit the transfer paper work with your $200 transfer fee. Transfers are not confusing; they are simple; finding the registered rifle, and or, trigger groups, are hard. You have to go through a Class 3 dealer.
You can't build a full auto "machine gun". That has been banned since the 90s. You have to buy one that is already registered. \
Do not under any circumstances, attempt to convert a HK91 to full auto; that is a major crime.
Just trying to keep you out of jail.

Sam thanks for the info the HK website was useful but I will have to read it a few times lots of info there .

http://dealernfa.com/product-category/machine-guns/

gonna run you about 35-40K