From my experience the internet is the cheapest place to get Touch-Up paint! Whether GM or aftermarket, just search.
Personally, I would start with a tough-as-nails base coat of POR-15 45004 Gloss black Rust Preventive Paint from Amazon (4.5 stars).
There is no such thing as cheap paint. To fix the rock chips in my Vibe, I thing I wound up spending around $45~$50 on a small 2 oz bottle of paint, some spot dapper sticks, 2 oz bottle of clear, and shipping.