To buy one hit up
http://www.gmnavdisc.com/.
Right now they are 160 normally 200 bucks. The DVDs can be copied easily enough but it's just a lot for effort for a very niche audience. the people who do make copies usually just post them on ebay for a quick 40 here and there. If you plan on keeping your ride for a long time I would suggest just buying another head unit. You will get more features and you won't get beat down by the GM we got you now tax.