Here are some great places to get cake decorating supplies.
I will add to the list as I come across others.
Pfeil and Holing: All cake decorating and baking supplies you could ever want, almost. Great shipping prices.
Boba Tea: I use bubble tea straws as dowels in my cakes. They are very easy to cut and strong enough to support a tiered cake. Works very well. Oh and they are cheap!
Global Sugar Art
Dallas Foam Cake Dummies: For all of your dummy needs :)