2006 Chevrolet Colorado where to buy correct thermostat

I don't think that is real important but you should be able to get it from the dealer.

I have changed out the entire cooling system, and sensors but it is still running too hot and almost red lining.... it is due to the bore size of the thermostat. Through research i have found that the truck will be running hotter due to the bore size and smaller valve inside the thermostat.... dealership can not find original part... was wondering if anyone on this site knew where to buy original thermostat online

I wouldn't bet on that being your overheating problem. You probably have a blown head gasket from the previous overheating.

I had system preassure checked...it passed....mechanic said cant be head gasket if it passed the preassure check...

Then I suggest a different mechanic be he is wrong.

How could head gasket cause it to run hot? Would i loose coolant level if it was head gasket, or see some sort of leaking around the head? How would i check Head gasket myself?

You can check it yourself. The best to check it is with an exhaust analyzer sniffing for hydrocarbons in the coolant at the radiator.

It can be pumping hot combustion gasses into the cooling system, superheating the coolant and causing air pockets. I'm not saying it is. that's just one possibility that can't be overlooked. Overheating can be difficult to diagnose and you need a tech that knows what he is doing. This is a very common issue in South Florida where I am.