Piping gel is like what you get at the grocery store but better, so don't try to substitute. The grocery store stuff is a lot more runny and gloppy.
If you have a Michael's or another craft store around you, I know Wilton carries it.
Piping gel can be used for writing or other decorating right on the cake in addition to the rice paper technique (which is fun!). You can also mix it with butter cream to make a decorating icing that's lighter in color (pastel) and consistency (good for writing).
-A balanced diet is a cookie in each hand