Did some Google searching as both my P5 cables have been worn up. I found what looks to be a decent cable. Looked up a forum or two and they mentioned a "Marantz PEC/P5 cable" which led me to this. Some more quick searching and I found the Japanese (It's Japan Amazon!) Amazon page for the PEC/P5 Upgrade Cable
The display price is 6300 Japanese Yen, which translates to roughly $81 USD. This is much cheaper than the Stefan Audioart P5 Cable which goes for $300.
The PEC/P5 cable looks to be of decent quality and not too expensive, but it is a funky blue that doesn't really match up with the deep black and silver of the P5. I don't have any experience with the cable since I'm in the States and Japan Amazon doesn't ship to the U.S. (or so I heard).
(A picture I snatched from a blog post)
EDIT: a picture
Edited by Bossatiger - 9/22/12 at 1:41pm