They are available for purchase seperately, though i don't know how well they're supported overall. Otherwise Adaptec always seems to fall short in some crucial way, leading most to buy other brands. Hopefully this product is better than the others were. I def wouldn't buy an HP just to get my hands on it, Dell does carry them though, and lots of coupons to boot.
I got it off Woot a long time ago (as evidenced by the age of this thread). It has worked great, but customer support is a bit lacking (you can probably blame poor sales as much as anything, not a lot of buyers, no reason to pour support time into it), and it's not discussed much in forums and such. A shame though, dual tuners, plus FM tuner plus two S-video inputs, it's pretty stacked.