7N silver and copper are available on the market, such as the following:
- Toxic Cables Mundorf 1% Gold Silver (on eBay)
- Moon Audio Silver Dragon (I think the purest version was discontinued for DIY though, only 4N now)
- Whiplash Audio TWag (Not available for DIY, although there has been an interest check by the MOT)
Copper (all OCC):
- Double Helix Cables "Nucleotide" (my personal favourite)
- Toxic Cables OCC (haven't tried it yet)
- Jena Labs Ultrawire (95% sure it's 7N)
There are also single-strand wires like Homegrown Audio's (solid core silver) and Clairvoyant Cables' (gold plated solid core OCC copper).
However, the 7N purity cannot be proven and is rather an estimate, as tests can only prove a purity of up to 5N as of today.
If you're looking for a certificate, Clairvoyant Cables offers one when you buy a spool of their wire. Other than their official website, there are often deals on eBay, so keep an eye open for those.
Hope that helps.
Edited by Benjamin6264 - 5/7/12 at 8:26pm