Most lighting retailers and industrial electrical suppliers will carry Advance ballasts - just match the wattage to your lamp. However, because compact flourescents really aren't that popular yet for 'mainstream' uses, you may not find the ballasts or socket clips in stock and the price might not be all that sweet.
There's also a potential issue of heat generation putting 55w lamps and ballast in a fixture designed for 18w lamps.
You may also want to make a serious cost comparison between buying the ballast and clips and a cooling fan and retrofitting your existing hood, versus just buying a new JEBO/Odyssea hood with a good ol' Chinese made dirt cheap ballast and clips and fans (which you can't buy separately). They're pretty cheap on EBay ... http://cgi.ebay.com/ws/eBayISAPI.dll...category=46314