There's a good deal on 8gb of Macbook Pro-compatible RAM here:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231294
Keep in mind that if you have 4gb installed, you actually have two 2gb sticks (rather than one 4gb stick and an empty slot). So if you do want an additional 4gb of RAM, you have to get two 4 gb sticks and replace your existing 2gb sticks (sell on eBay, maybe?).