well almost all silver wire you're going to find is of fairly high purity and that's because it's either used in jewelry, medical, or scientific disciplines where high purity is required...especially when it's in teflon.
You can look up New England Wire & Cable on the web. They're an actual manufacturer of bulk wire and I believe they also do silver. You can contact someone there via their toll free number about what your requirements are.
Another source is
http://www.a-msystems.com/physiology...silverwire.asp
and if that link doesn't work, i'm sure you can renavigate the site (I didn't check it).
Silver, being a precious metal, varies in price.
Post back if you will what you found for prices.