Your paint code should be on the RPO build label in the glovebox. Bottom line on GM's label are color codes. A lot of dealerships don't carry touch-up paints anymore unless they have a body shop too because of shipping costs associated with them due to the necessary documentation. I find most parts stores now only carry the most popular colors and generics as well.
Once you know the paint code, however, you can order Duplicolor is any available color code through Amazon and others, but I personally have had much better luck with a really good match having it mixed (I have a local automotive paint distributor) or using paintscratch.com. Being that I have a real, ummm..., "particular" personality I'm all over my vehicles and camper looking top-notch and let me tell you laguna green metallic and windy seas blue pearl are tough to match! Thank goodness my RAM is bright silver metallic (it's easily matched).