One option not mentioned and maybe goes without saying is Amazon.com, the US site. I have come across some great deals on Amazon because you don't have to deal with bidding, you either like the price or you don't. Again maybe goes without saying, but here you will especially find US release soundtracks that may be hard to track down otherwise. Japanese releases also appear from time to time, sometimes at very high prices, sometimes reasonable. Amazon.com seems overlooked in the wake of ebay. Also you are only paying domestic shipping (if you are in the US), so it doesn't drastically inflate the 'sticker price' like imports do. There was a seller on there who had an amazing collection of soundtracks up for sale at great prices, but they suddenly disappeared a few weeks ago
Come back!
I generally order from one of the websites already named, and Amazon.jp the most, but here are some others I have chanced upon. Fair warning, I haven't used their service and can't vouch for them.
http://www.gamebaz.com/?a=s&c=audio&s=cd-stv
http://www.jap-sai.com/Index.htm
http://www.cduniverse.com/default.asp?style=music
http://www.amedis.co.jp/ <------***Japanese language***
Yahoo Japan auctions site, 'the ebay of Japan':
http://list3.auctions.yahoo.co.jp/jp...-category.html
To use Yahoo Japan auctions you will need a middle-man service like:
http://shoppingmalljapan.com/
http://www.celga.com/ <------***I have used and can vouch for them***
http://www.cdjapan.co.jp/request/shopping.html
Sorry if anything is a little outside the scope of the posters question, or redundantly basic info :P