OEM from chevy, or thule/yakima. i have had good experiences with thule at one point on a vw gti. the aftermarket options will prob be cheaper then from the stealership unless you can find used oem's on a enthusiast forum for chevys or craigslist.
+ 1 on http://www.orsracksdirect.com/, good pricing and service. the best racks out there are the whispbar racks. they are strong, lightweight and most of all quiet. no need for a lame fairing to control air noise over 60mph.