If you want to search for The Brotherland, Moskow or anything else, use the following search box.
When it comes to current music, I think Amazon is hard to beat, but for The Bolshoi, you can do better. All you will get here is the CDs, and the price is not as good as eBay. But if you are concerned about using an auction site for any reason, this is safe! You can also find Trevor's solo stuff here, but might as well get that below. No Brotherland, Moskow, Celtic Soul or Rathkeltair, though!
Emperor Penguin Recordings - Trevor Tanner
If you want to buy Trevor's solo album(s), you can get them here -- straight from the source -- if you'd like.
CD Baby - Rathkeltair "Everybody"
Rathkeltair's first studio release is available through CD Baby (the band's official source other than live shows).
CD Baby - Rathkeltair "Live"
Rathkeltair's first release -- a "live" effort. Also available through CD Baby.
Digital Downloads - iTunes
Unfortunately for non-iPod owners, it's tough to find legit downloads for the band. Even searching Kazaa, Limewire and other P2P services yield only a few results. You are better off buying a CD and burning it yourself, IMHO.