Go to the my chevrolet (my.chevrolet.com) site. You can see stuff about your vehicle (once you've created an account) and there is also accessories available. Talk to the parts manager at your dealership as they may offer discounts for accessories for new vehicles.
gmpartsdirect.com is a good source for discount parts. Talk to the sales manager to cut a deal on accessories for the vehicle you just purchased from them........most will give a 10%. and compare prices pls the inconvenience f waiting.
But for any online purchase you also have to calculate the shipping costs for larger parts into the cost and whether or not tax is collected for your State as opposed to paying more at the dealer plus State tax but no shipping costs, eg. for my Silverado sports bar:
Saved money and time buying at the dealer even though a lot cheaper on line ......Dealer cost $900 + $45 tax = $945 vs Online $864 +$125 shipping = $989.
Also remember than many accessories do not come with instructions just a sheet of paper indicating they are available at the dealer's parts department......dealer may be more inclined to print off the instructions from parts you order from him without a hassle and worth it for a DIY install. Eg. Ordered the running boards from he dealer which I planned to install, but after he printed the instructions decided he could do a better install with an air powered riv-nut gun and lift than I using a kludged bolt, nut and washer to tighten the riv-nuts properly and multiple jacks on each side for the wheels off install