This is what I just did buying my 18 1500 CC. I built the truck I wanted on GMC's site and filled out the "have a dealer contact me" form. 4 dealers wrote back in an hour, all had at least 1 if not more of what I was looking for. The GMC website will also find trucks in inventory and indicate if they are exact matches, or what the differences are.
There was one dealer I really liked. They had the exact trucks I wanted except they all had had a moon roof, which was a deal-breaker for this tall, fat guy... They found an exact match (minus moon roof) at another dealership, had it swapped up to their dealership, and we did the deal.
I would say you'll need to HURRY to find a 2018 though. I was trying to get in on the GMC rebates specifically for the SLT Premium Plus Crew editions. I ultimately got the exact one I wanted, but they were becoming scarce a couple weeks ago when I bought mine.
Bottom Line:
Find a great dealer and have them do the work, it's not as hard as some make it sound...