Youd have to special order them...... Either through a dealer, or get them off the internet (gmpartsdirect.com)
Installing them is VERY arduous.... You have to take off the seat covers, and then replace the pads..... I dont know how hard it is, but it would defintely be a DIY, since its not hard, but very time consuming...... Youll also need to get some upholstery tools (like a hog ring fastener and stuff like that)......