There's a tutorial for making conductive glue here:
which mentions that you can buy
"Carbon Graphite, fine powder- Available in larger quantities at http://www.elementalscientific.net/
Available in smaller quantities at your local hardware store. It's called lubricating graphite and comes in small tubes or bottles. The brand I used is called AGS Extra Fine Graphite, but no doubt there are other brands that will also work."
I haven't tried it, but it should be possible to mix fine carbon graphite with screen printing medium. I'm interested in mixing it with block printing medium for lino block printing, myself.
You can find conductive paint (which they say can be screen printed) as well as several tutorials for use here:
There should be some ink companies. One ink company I am working with is http://www.conductives.com/.