Yeah, they're out there, but too expensive last time I checked. Unless you absolutely must have more than 4.35 GB on a single disc, you're better off sticking to single-layer DVD-Rs.
Edit:
here's some. They have really dropped in price, so what I said is no longer valid. Wow. Big drop. Last time I checked, they were like $10 apiece. Now it makes more sense, although you still get over twice the GB/dollar with SL discs.