Check the Ibanez website. They sell the parts there.
http://www.ibanez.com/parts
Find the model number (EDB600, not sure what color there are 5 options), and then there's a parts list and knobs are near the top. If you can find an Ibanez dealer, they can get ahold of the parts for you. If not, PM me and I'll talk to my local guitar center and then see if I can ship them to you.
Additionally, you check online for regular knobs and mini knobs. It might be cheaper and you have a plethora of colors available.