Hydrocarbon standards are commonly available through the suppliers of GC consumables or vendors of standards. Depending on what you need, you can find these for environmental analysis, for petroleum analysis, and I suppose for other applications. If you do not have a list of favorite suppliers, I would suggest looking at the web site for a scientific conference related to the type of analytical work you wish to do, check trade magazines, or check the web site for a major supplier conference like Pittcon.org.
Of course, if you are just looking for normal paraffin’s for retention time markers, a bit of paraffin wax (available at a grocery or hardware store) in a bit of hexane works just fine (and is inexpensive).