All you need is to be handy with a soldering iron.
Take your FlashTrax to Radio Shack, and tell them you need to find the right size plug to fit the hole where the charger plugs in (says "5V"). They have a bunch of connectors on a ring and you can try them to find the right one. Buy a package of the right ones... they come in packages of two. About $2.
Then get the black plastic battery holder that holds 4 D batteries. About $3.
You also need some cable to connect the battery holder to the plug... 18 gague is plenty. You should be able to find an inexpensive coiled power cord for about $8. (see RS part numbers 270-031 or 270-1592).
Cut the ends off the new cord, and solder one end to the battery holder, and the other end to the plug for the FlashTrax.... make sure the NEGATIVE from the battery holder goes to the outside contact of the plug.
Now you can use any rechargable or alkaline D cells now. Get the heftiest ones you can (i.e. highest Amp-hour rating). These are the ones I use (7Amp-Hour)