This is probably a stupid question, but...

Oxidized silver is definitely something that you can purchase. There are many websites that offer them and I've included just one link of many. You can also check out any flea markets as they often have silver jewelry and may offer good deals on oxidized silver. Here's the link: http://www.vikingtrader.net/chains.html

hey you can do both you can buy it oxidized or do it yourself. A couple of really good sites to check out are firemountaingems.com they have near everything and the more you buy from them the better discount. Also another place I really like is monsterslayer.com great prices on silver and copper wire. There are a couple of methods used to oxidized silver yourself one is by using liver of sulfur another is buy using the yolk of a hard boiled egg. I have never done either but unless you are going for a very specific look that you can only do yourself I would stick to buying it after using either diy method your gonna have to polish the parts you want shiny by hand or with tool (dremel etc) which is gonna take some time. Hope this helps some.