where to get shorting plugs?

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.

I did as Jgiacalo described. Chopped the cable off leaving about 1/2", stripped the insulation and twisted the wires together. I didn't bother covering them with tape, I just twisted the exposed wires into a little pigtail real tight to the cap. You can do the same for non-shorting plugs, but in that case it is important that the wires are NOT shorted, and that they be covered so they don't short.

(http://www.cardas.com/content.php?area=oem&content_id=7&pagestring=RFI/EMI+Protection+Caps) read the following and pay attention to what is said about shorting outputs:(http://www.siteswithstyle.com/VoltSecond/Shorting_plug_rca/shorting_plug.html)

Thanks all, very helpful. I learned that Dedicated Audio sells the Cardas ones (Cardas produces both caps and shorting plugs).

seandtaylor99  +1  His suggestion works great for me. Noticeably cleans up some HF hash. For the 6 unused inputs on my preamp, instead of making stand-alone shorting plugs, I made 3 pair sets. In other words, I connected/shorted the bare wire from two RCA plugs together - and installed each pair in their respective left and right channels. I didn’t solder - just twisted the wires together and taped them. I was pleasantly surprised how easily I could hear the improvement. Defiinitely, DO NOT use shorting plugs for outputs!  YMMV