First thing you need to do is to get the potentially contaminated oil out of the block! Antifreeze will eat up your bearings very quickly! Draining the oil needs to be your top priority!
A lot of us have used the Dorman Upper Intake. I know that Advance Auto carries it. You can also try Napa. Fortunately, this is a common part so finding it shouldn't be too tough. Plan on spending about $130. I'm not sure if the gasket kit comes with it. If not, I recommend buying Felpros.
Since you're in there now is the perfect time to do the lower intake gaskets. It should only take you a couple more hours worth of work. I think you'll be shocked when you see how bad they'll be degraded. I also recommend Felpros for those. And when you take off the lower intake, replace the 90* coolant by-pass elbow between the LIM and the belt tensioner. Over time they're known to get weak and brittle.
Keeping everything clean is the best way to be successful at the job. If you don't already have one, pick up a Chilton'*. You'll also need a ft-lbs. torque wrench.
Some say that you can use a 180* thermostat to prevent this in the future. I didn't change out my OEM 195 because I didn't want to give up the gas mileage, plus I'm not convinced that'* going to help any. You can do it if you'd like, though.
And for ease of reassembly, it might not be a bad idea to pick up a new o-ring set for your injectors. Being 10 years old they might be tattered. Those are a dealer only item (about $6), as is the coolant by-pass elbow ($7 from the dealer).
Any more questions..post 'em! I've done the job twice. Also, search the forum. You'll find TONS of information on this job. It'* happened to most all L36 owners.
Oh, and welcome!