Aftermarket is the way to go. I never buy anything from GM for any of my GM vehicles. After 10 years, even the jap motorcycle manufacturers discontinue alot of parts.
They want you to buy new every 5 years or sooner. That's why most vehicles built today self destruct around that time frame. They want you to get so sick and tired of failing inspections & breaking down that you give up, walk into the dealership grabbing your ankles, ready for a $500/month raping. It's an ingenious system they've devised with the governments help (inspections). Like I said - It ain't about safety or clean air. It's all about money. More tax dollars coming in from unnecessary repairs. The EVAP system is the biggest racket ever devised by the automotive industry.
I can't count the times I've fallen over dead from one whiff of gas fumes ....
I PM'ed you a source for that neck, btw. Stay out of any dealerships.