Then you need to look at GM Accessory's. The are exclusive to GM's design and not available under another brand name.
I am not endorsing this site, use it for reference. I have the GM Chrome 3" Round Tubular Assist Steps on my GMC and they are high quality and fit perfectly. They do cost a lot more but that's because they are worth it. Shop around, prices vary and be careful with low prices and outrageous shipping and handling costs. By time I factored in the shipping and wait to get them, the GMC dealer was the best value. If your err and order the wrong pars online, most will not allow you to return them or if they do, make you pay a restocking fee + return freight so keep that in mind.