My hero 2 came from ebay. It was a seller called maxproaudio. I was very nervous about buying from ebay but it all turned out perfect. Having said that about 4 months after buying it it started to freeze. I contacted gopro directly who asked me to send it to them in America. Within three weeks i had a brand new camera delivered so customer service wise it was excellent. All i had to produce (email) was a copy of the ebay / paypal transaction which showed it was a new purchase and not second hand.
In terms of mounts and having a look at the camera bits before buying try popping to the clothes shop "animal" as they stock a fair bit for gopro. Well they do in canterbury anyway.
Failing that. These chaps have given me some excellent advice and this is where i got all my extra bits from http://www.shredheads.co.uk/index.htm
Last thing is that the sticky pads are bloody strong. I use one on my waterskis and don't bother with a tether now. Make sure you buy a floaty back door attachment so worst case scenario it floats.
Hope that helps