Try “Sparkfun.com” they will probably have something.
Depending on how handy you are, there are plenty of circuits out there for ‘do it yourself’ amplifiers …
"People sleep peaceably in their beds at night only because rough men
stand ready to do violence on their behalf." - George Orwell
I noticed that on the Audio How-To page at http://wiki.gumstix.org/index.php?title=Category:How_to_-_audio, it says that an amplifier is needed for Expansion Boards without a stereo headseat jack in order for a microphone to be added.
Could anyone point me in the right direction as to which amplifier I should buy? I am using a Palo35 Expansion Board.