ebay, chainreaction (in the UK, so shipping takes a while), jenson (price matching), pricepoint, treefort bikes.
I just do a google search for the item I want. Then I click the "shopping" tab above, then sort by price. Sometimes a bike shop will be blowing it out at a cheap price, and this is the best way to find it.
Make sure you get the right steer tube size (1.5", 1 1/8", or tapered), and make sure you get the right axle setup on that fork. I'm betting you have a 9mm qr up front. Forks come in 9mm, 15mm, or 20mm axles. I really prefer the 15mm front axle, and since you're paying for a new fork anyway, it may be worth upgrading to a 15mm axle. However, if your front hub is a 9mm qr, you're going to have to either convert the front hub (which may not be possible) or get a new front wheel.