1/4" (inch) Black Hole Plugs - WidgetCo.com
I use these in 3/8ths 5/16ths to plug any potentiometer holes in guitars with pots I am not using. You can find them at Ace Hardware.
Mesa dealers sometime have slightly fancier ones in 1/4, since Mesa cabs ship with them to plug the inputs in the back of the cab not in use IRRC.
[VIDEO]http://www.amazon.com/Black-Plastic-Flush-Type-Plugs/dp/B0040CTVRW/ref=sr_1_59?ie=UTF8&qid=1376189067&sr=8-59&keywords=black+hole+plug]Amazon.com: 100 1/4" Black Plastic Flush Type Hole Plugs 1/2" Head: Automotive[/VIDEO]
Pretty much exactly what Mesa uses.
Or if you want the corksniffer factor, here is one specially designed for audio jacks!
[VIDEO]http://www.amazon.com/Generic-Headphone-Cover-iPhone-LifeProof/dp/B00C5ZSNOS/ref=sr_1_21?ie=UTF8&qid=1376189044&sr=8-21&keywords=black+hole+plug]Amazon.com: Generic Headphone Jack Cover for iPhone 5 LifeProof Cases: Cell Phones & Accessories[/VIDEO]