try google, but not as GT31 batteries try something like this.
www.globalsources.com/GPS-battery/portable-lithium-battery-1183707279p.htm#1183707279

Hello Windfoiler,
As the USA LOCOSYS Distributor, I had a inventory of genuine replacement batteries. I need to look as see if I have one left.

Otherwise use this link to get a battery from Sparkfun. I have been told by a number of riders that this is the correct battery, including the connector. Try this link:
www.sparkfun.com/products/13813#

Good Luck
Craig Bergh
The Midwest Speed Quest
www.midwestspeedquest.com