I recently bought a 2000 Subaru Outback wagon, and i'm looking to buy a kayak carrier for a trip i'm taking in two weeks. i don't know where the best place to look for them is, or where i can find a cheap-ish one.
Stick with Yakima or Thule - they are the two top brands. If you live in a big city you may have a rack specialty shop where you can look at some, or REI carries a bunch.
I don't know about Thule, but Yakima prices are pretty much regulated by the factory and you won't see much savings from buying online and paying shipping, vs buying local and paying tax. OTH, REI regularly has a "20% off one item, everything in the store" sale (which really isn't everything) but this does include Yakima products, so you can save a fair amount on a single expensive item. Since you have a short deadline, you may be stuck with paying near MSRP
If your kayak is really heavy (over say 75 pounds) then you need to upgrade the crossbars, since the OEM ones are not especially strong and are held on with small self tapping screws, and the curve makes positioning stuff so that it is level problematic...... If it's a dinky one man boat, then the OEM bars will be fine. Take a look here:
To add to what John has said about Thule and Yakima pricing, while it is pretty much set in stone you can occasionally find sales. I know for certain that once a year REI puts all Yakima and Thule on sale 20% off. I think Thule is in the spring and Yakima in the late summer, but I'm not 100% on the timing as it's been quite a while since I last needed anything rack related.
How many kayaks are you taking? If you are only taking 1 just strap it to the roof racks that are already on the car. I do that frequently with my lake and white water kayak. If you are going to take 2 you will need to get some wider rails. That is assuming you have a standard plastic kayak. If you have something with a rigid hull that doesn't flex when you tie it down then disregard the above.
The standard kayak carriers are really not needed unless you are looking at spending money or you have a high dollar fiberglass kayak.
X2 on this, i have a 13.5ft Wilderness systems Pamlico double, i put the foam pads made for a kayak on my factory rails and strap it down, been doing it for years, my kayak is about 90-100lbs loaded down.
my brain is in the prosses of thinking up some kind of roof basket that can double as a kayak holder of some sort.