WalMart plain white. It's like 12 for $5. Do have to wash them hot to clear out some of the sizing, though.
The local outlet store occasionally runs a good price on nicer ones.
For neck rags and bugout gear, I just watch for cheap shemaghs online and tack one onto an order when I find a good price from a vendor I'm buying something else from anyway. Makes a pretty good scarf in the winter, and having the option of wrapping my head against the cold and/or sun is handy too.