Hot Bodies USA http://www.hotbodiesonline.com
They have a good amount of spares you'll need. Some of the hop ups aren't in stock at the moment, but it's always worth a look.
Stormer Hobbies http://www.stormerhobbies.com
These guys are a very large online retailer that have just about anything and everything in the US. They also have a pretty decent stock of HPI/HB parts.
If those both don't suit your needs, go with STLNLST's suggestion and try RC MARKET at http://www.rcmarket.com.hk
He always has most of the goodies in stock, and has very good service. Shipping is a little more than the other two because they are based in Hong Kong. Always a good source for parts though.