I am looking at the same thing. I called Westland (the manufacturer of the cover) to ask the same questions. According to the manufacturer, Sharkskin does not breath well at all. They said this is only a problem if you live in a humid area. The lady I talked with recommended using a de-humidifier (like damp-rid) packet under the cover. I think you can buy them at Home Depot. She also mentioned venting the Sharkskin as a solution. She also said the Sharkskin is not as good as Sunbrella for towing - but Sharkskin is more water repellant then Sunbrella during rain.
Overtons does sell Westland covers. So does http://boat-cover.net
Another place I found is http://www.coversearch.com/
I am not sure if coversearch.com sells westland covers though. I don't know the reputation of any of the above places. Hopefully, somebody else can help us out there.
There are also other re-sellers on the net. The Westland customer service person I talked with also said they are the OEM manufacturer for boat covers for some of the wake board boats.