The Nikon- and Canon-branded battery grips are insanely expensive. What alternatives are out there, and where is the best place to buy them?
I highly recommend Zeikos battery grips. I have the Zeikos grip for my 5D Mark II, and I could not be happier. It matches my 5D2 perfectly, with no flex. With two batteries, it gives it enough heft to match my bigger lenses. It has vertical shooting controls (which I never use), and sits firmly attached to my 5D2 body.
I bought it from Amazon, paid $90.00, compared to the $250.00 that Canon's brand grip cost.
Regarding reliability, I've taken it with me now on several shooting trips: Greece, Holland, Germany, Spain, France, Sweden and Finland, and it's never had a single failure.
I've not handled the Canon 5D2 grip, but the Zeikos feels identical to my canon brand 40D grip.
I believe they also make Nikon grips, but I can't speak to them, as I am a Canon shooter.
They are definitely a chinese knock-off brand, but the quality is surprisingly good, and if you need a grip and are looking to save money; you can't beat it.
Now, just a word of caution: while I recommend 3rd party grips, I can't recommend 3rd party batteries. I've read enough horror stories to stick with Canon's over priced batteries.
I have used the Opteka brand which was outstanding value for money. For example for the Canon Digital Rebel series, check this out:
For the price (at the time of writing), and given the inclusion of two 1800mAh batteries, it's pretty hard to beat.