A/C o-rings, where to buy?

[treed2 at wsu.edu (T. Reed)]

So I'm going to try to reassemble a working A/C system from the pile of 
parts in my garage, and I want to try to do it right if possible.

That means pulling a vacuum before charging, getting a new 
receiver/dryer and possibly an expansion valve (it's what plugged 
catastrophically and led me to remove the A/C "last time"), draining as 
much of the old oil as possible (is there a good way to do this?), and 
getting an o-ring kit to replace as many of the o-rings as I can find.

The only problem is.. I don't know where to get an o-ring kit! Is this a 
standard auto parts store item or something specific to VW?

Any advice/pointers would be appreciated..

-Toby