If you mean one with a fan, Targus makes one that should fit the 17". Know a couple of people using them on PowerBooks (15"). Very, very quiet. They also make a model with a USB hub built-in, if you don't want to lose a port to the cooler.
If you're just looking for something to support it, on desk or table, I've been using a CoolPad (also Targus). Not sure if there are still two models of it, but I'm using the one that has an adapter for larger notebooks, that fits across the back support pads. You can also adjust the height [of the rear]. Oh - and it swivels.
For actually putting it on you lap, I think the one with fans would be much more comfortable, as it's more like a small desk. The CoolPad just isn't wide enough for me to use it that way.