If you google 'toyota parts' or 'toyota accessories' you will get a ton of hits for dealer websites that sell the Toyota accessories at a discount. Most sites allow you to put in the year, make and model to see only the parts applicable to your vehicle (or that they carry). Most have the part numbers there, or sometimes you can add the part to your shopping cart and see the part number there.
I did this for several sites and put the part numbers and prices into an Excel spreadsheet when I was shopping around for parts. eBay is also an option once you have the p/n, just make sure they are new and unopened so that you aren't getting a part that doesn't work or is missing parts...
Also, there are a few threads here re: the satellite radio install, a very good one is here:
How to: Sirius Radio installation. There are 2 parts you need whether you go with Sirius or XM - the receiver itself and the mount kit. As for installation, the factory instructions have you mount the receiver under the driver's seat; however, many have found that they can mount it behind the glovebox and also do it without having to remove the headunit...
Good luck!