It's strange because they sell glycerin in a lot of places but the first few times I went looking I had a hard time too! It's out there though, so just keep your eyes peeled and try looking in different sections.
It should be easy to find at a drugstore/pharmacy like Walgreens or RiteAid, look around for a small dark brown plastic bottle usually around the area where other first aid supplies are at. The bottles can be inconspicuous and easy to miss.
You can also try looking at the health food stores, Vitamin Cottages, Whole Foods, places like that. Of course there are plenty of places you can order it online too, but there are absolutely local places out there that have it so keep looking and good luck!