For quite some time you could get the dipsticks from the dealer. Seems that they are slowly being disco'd as well, but it can't hurt to call and ask. If they say they are discontinued, ask for the part number. Then plug it into someplace like gmpartsdirect.com
If they can't or won't give you the part number, they are lying, which unfortunately speaks to the general incompetence of many parts counter folks, at least GM.
If they say its disco'd and can give you a part number, it's a good bet they are right. For some reason dipsticks seemed to be one of those reasonably priced items from GM, whereas things like a disk brake rotor for the same vehicle would be outrageously expensive.