For generic stuff in the States you can find parts at Treats. They carry mostly moped stuff, but they can get a lot of aftermarket things for these bikes, simply just ask them. They also carry tires for these bikes. Treats is usually my first go to with regards for things, they are very friendly, and have always treated me awesome.
For OEM parts I've found that EasyParts is one of the best sources. They are easy to work with, and are able to get many parts I've not found in other shops. Very cool online shop.
For hop up parts MaxiScoot seems to carry a lot of nice pipes, top ends, and other items that come in handy when you are building a project.
Aprilia OEM factory parts can be sourced from AF1, and they seem to actually have some Derbi parts in their catalog. AF1 has the front Ferodo pads for the 98-2004 GPRs, these are the best pads I've found for these bikes. This is the #1 Aprilia dealer in North America!
TRT the makers of the 2Fast products. This is the source for the engine parts on the Red Rocket. These guys went all out with helping me prep for the Japan adventure.
Quad Lock makes the super cool mounts as seen on the Red Rocket, and the DRP. Use the code GPRCAMP to receive a ten percent discount!
For Brembo masters I've found that Kurvey Girl has nice prices on the parts we need, like the RCS14—which you can check out in the Red Rocket thread. I’ve found them to be quite kind to their customers—this is where I go for my Brembo parts.
I've found that the Bolt Depot has a decent selection of bolts for these bikes. Stainless socket caps are available for most needs.
I'll update this thread from time to time as needed.
Please try to keep this thread on topic!