SecureLine 3/16" diamond-braid cord by Lehigh is a favorite of bubblers.
LABELING & AVAILABILITYEditThis cord comes labeled a few different ways sometimes labeled as simply "Cotton Diamond Braided Cord" sometimes as "Diamond Braid Cotton Clothesline". The label color and layout changes periodically.
(August 2017). It is currently available on Amazon and is often available in hardware stores. In the past, it was available at WalMart (though you might have to look in a few different sections of the store to find it) but recent reports indicate that WalMart may have stopped carrying it.
This is a great cord but requires a little preparation. The first thing that you need to do is to remove the center core. This cord works terribly if you forget to remove this.
Once you have removed the core, you should let the cord soak for a while in water and dishwashing liquid, rinse it and soak again. This removes some 'sizing' that inhibits bubbles. The cord should work well at first but will get better as it breaks in.
REMOVING THE CORE - VIDEOEdit
August 2012. It has been reported that the SecureLine cord is becoming harder to find and is no longer stocked at all Walmarts.
Update 09/10/13: According to one person, SecureLine cord is being replaced by the housebrand "Mainstays". However, as of that date the SecureLine cord is still found at Walmart in the SF Bay Area and perhaps elsewhere.