Amazon, Amazon, Amazon. then check out Buy.com, Fry's (online and B&M), there are a few others, I believe.
I don't buy any BD/HD DVD unless it's $10-15 these days. (well, for the most part)
stay tuned to the special sales threads in AVSforums HD DVD forum. there's almost always a deal going on somewhere; although admittedly, most of the deals are for BD.