I'd first recommend reading the (you need an account to see links) over on the "(you need an account to see links)" section of the forum. You can then browse that forum - a lot of sellers have lists of things they've currently got up for sale. I'd recommend browsing a few of those threads first to see if anyone has what you're seeking.
(And iTrader is a great way to see a user / seller / buyer's stats and feedback. I like to give it a quick look before I buy from a new seller for the first time. You should also double-check if a user has the "Trade Banned" userbar (you need an account to see links). If they have that userbar, you shouldn't be transacting with them.)