where to get shorting plugs?

Here's some from the Tweek Shop:

http://www.tweakshop.com/Shorting%20Plugs.html

If you want to short unused RCA inputs then what I've done is to take the cheap disposable interconnects that come with most components, chop them in half and then short the conductors (with solder) and put some shrink tube on the exposed end. Presto: two sets of shorting plugs from each interconnect that would otherwise be disconnected.

It's not hard to solder them because you are actually TRYING to short them. It would be much trickier to solder the conductors without accidentally shorting them !

I got mine from MCM, they work just fine and are inexpensive.

http://mcmelectronics.com/search.aspx?C=&K=rca%20caps

Making your own isn't all that hard as Seandtaylor99 described. Or, you can just twist the wires together and cover them with electrical tape. Not as durable as soldering but wokks just fine and lots cheaper that 'custom' off-the-shelf plugs.
shorting plugs reduce noise you didn't even know was there.