Not to be cryptic, but pin stock is where you find it.
In other words, while it isn't exactly everywhere, it is more common than one would first think.
Brass pin stock is probably the easiest to find. I get most of mine through my local Ace Hardware Store. Most of it they carry in 3’ lengths and sell for less than you will pay for 2’ of the same brand from other sources. I have also found a good supply of brass & stainless steel pin stock at my local True Value Hardware stores as well.
Many hobby stores, both online & local, will carry brass pin stock, sometimes stainless steel and occasionally a limited supply of nickel silver.
The best variety of sizes of nickel silver pin stock that I have found is at Jantz Supply. I find their website somewhere between abysmal and impossible to use, but if you call them and ask for a print catalog you can see the sizes they have.
Here is a link to a thread I posted some time ago on sources for pin stock:
http://www.allaboutpocketknives.com/kni ... 37&t=23455
When I must have a certain size pin stock and don't have anything close to the proper size, I will turn down a piece of larger stock by chucking it in the drill press and taking it down with a file & sandpaper to the proper size.
Another method is to chuck the stock in a drill and spin the stock against a running belt sander. That takes it down fast, but it easy to take it down too far unless you are careful. It takes some practice to get proficient at this method.
Hope this helps.