there is a place in florham park, new jersey who has .99999 this is what I use. the purest silver on earth., in this century. email me for address ginseng777@hotmail.com

Nord,
This site makes some good points on the purity of silver, basically saying that water purity is more of a concern than silver fineness. IF you can find this elusive 5 nine silver, (I can't find it at all by searching google) you will probably pay about $200 for what you need. I would just stick with 4- 9s.
Ibis

http://www.perutechnologies.com/pdf_files/csLindeman.pdf
Purity of Silver
The quality of your finished product depends entirely on the purity of the water you start with and the purity of the silver you start with. Most of the current literature suggests that only 99.9999% pure silver can be used. Most home brew systems use 99.9% pure silver. So, what is the difference? To find out, I contacted Academy Metals, a company in Albuquerque, New Mexico, that produces commercial silver. The total allowable impurities in 99.9% (.999 fine) silver is 1000 ppm or 1 part in 1000. These impurities and their maximums are 1) Copper, 800 ppm, 2) Lead, 250 ppm, 3) Iron, 200 ppm, and 4) Bismuth, 10 ppm. This product is readily available in wire form and costs about $3.00 above the market (spot) price of silver. When this product is used to make electro- Colloidal Silver at a concentration of 5 ppm, the total impurities from the silver drop to 4 ppb (parts per billion) copper, 1.25 ppb lead, 1 ppb iron, .05 ppb bismuth. With all allowable impurities at these low levels, there is a reasonable argumentfor not being concerned. Still, sometimes small things make a big difference. 99.99% silver (.9999 fine) has total allowable impurities of 100 ppm of the same metals in the same ratios, and costs (in wire form) between $50-$90 above the spot price of silver. 99.999% silver (.99999 fine) has total allowable impurities of 10 ppm, and in wire form costs about $250 above the spot price. 99.9999% silver, in wire form, costs more than gold and is very difficult to find commercially.

In one sample of 10 ppm Colloidal Silver we sent out for total analysis (made with 99.9% silver electrodes), the primary impurities found were: 1) Sodium, 470 ppb, 2) Calcium, 260 ppb, 3) Manganese, 70 ppb, 4) Potassium, 50 ppb, and 5) Magnesium, 24 ppb. Since none of these impurities could have come from the silver, it suggests that the purity of the water should be of greater concern to the person making their own Colloidal Silver , than spending extra money on purer silver."

surepure dot com Fine-Silver-Wire-99.999Percent

THIS IS 99999 SILVER RODS OR WIRE FOR Colloidal Silver IN USA WHEN THERE ARE 5 NINES THEY CALL IT ATOMIC