Does anyone know a vendor who sells these for the 2010 RX8? I'd like to get a setup with certain spring rate.

I'm having trouble finding part numbers on Bilstein's website.

 

good-win-racing.com and tirerack.com both have them. I expect a bunch of others will too.

What are you trying to achieve with the spring rates? If you want custom, I would recommend taking a look at FCM: Fat Cat Motorsports - Miata 06+ / RX-8 04+ custom monotube coil-over assembly
if you have not already. It will cost a bit more, and can take a while to build, but you'll end up with a complete solution.