If you are not considering a public release, you can make use of some excellent sprites from the Civilization 3 modding community. They already have hundreds, if not thousands, of tanks and other military vehicles sprited in the 9-direction isometric/dimetric style that you can extract and recode into a grf for private use.
There are several Sherman and PzKpfw III variants available, for instance: this
To get these into OTTD, I would download the vehicle of choice, download this program
which will let you view & extract the sprites, open one of the vehicle's .flc files using that program, then use the Export tab to save the sprites as a .pcx. At that point you'll have some sprites to work with, although you'll need to resize them and convert the palette
over to the TTD standard
, of course.
For coding the sprites into a usable grf, I defer to the excellent NML tutorial