Fairly easy to find at most rural stores, Fleet and Farm, Tractor Supply, and some auto parts stores too. Most brands of 6v auto storage batteries are good.
You will wish to purchase a “Group 1” size 6 volt battery, as this is the correct size and terminal placement for the Model T battery tray, and for easy connection of the negative (-) terminal ground to frame strap.
You will want a fresh Group 1, 6v, capable of 650 cold cranking amps.
Just make sure your frame ground strap is new or like new, as the woven kind can get corrosion at the terminal lug. And be sure to clean the frame spot where the strap is bolted to a bright finish to have best ground.
Then the positive cables to the foot switch and starter motor need to be a size “00” cable. Parts vendors for the Model T have these.
Don’t use a modern auto parts store narrow diameter 12 volt cable, as that small gauge cable can’t pass the current necessary for the Ford starter motor.