As several folks have stated, Grainger is a wholesale warehouse supply company that does not normally deal with the general public, however if you walk in with cash they're likely to take your money.
Note - These "launch rails" are an industry standard aluminum structural extrusion made by several dozen extrusion houses, and they are all supposed to be interchangeable.
80/20 is one of the major US suppliers of this type of extrusion.
http://www.8020.net/
You can find your local distributor here. They also sell overseas so the line that Grainger gave JAllen is a crock.
http://www.8020.net/PostalLookup.asp
If you have an 80/20 distributor within driving distance you can save on shipping by picking it up, although they can usually get you a pretty good deal on shipping.
One of the best web suppliers of all things useful is McMaster-Carr
http://www.mcmaster.com/ and they stock 80/20 extrusions. (See attachment.) You should be aware that the 4' and 6' lengths are much cheaper to ship than the 8' lengths because anything over 6' is considered oversize by UPS. Since MMC delivers their orders by truck each night directly to major UPS distribution centers, UPS ground delivery will get your order to you next day in many parts of the country if you live within several hundred miles of a MMC warehouse.
Bob
View attachment mmcp1888-2009.pdf