I usually get evergreen - if you're starting completely from nothing, check out their odds & ends pack. A great value.
http://www.amazon.com/Evergreen-Scale-Models-White-Sheet/dp/B0006O5I20/ref=pd_sim_21_3?ie=UTF8&dpID=41iyvijJVBL&dpSrc=sims&preST=_AC_UL160_SR83%2C160_&refRID=04MVJFXW04VZQP9FV27Y
Ask around, some of your
FLGS should have a display too.
The evergreen stuff is good for a base, textured plasticard is nice for the exterior - try ebay.
http://www.ebay.com/itm/ABS-Embossed-Plasticard-A4-DOUBLE-DIAMOND-Tread-plate-Styrene-Plastic-Sheets-/281484761974
For treads, you can try to make your own (which I find a pain), but robogear has great grot tank bits. Try ebay for robogear varan, or hoard o' bits.
Honestly, you could do worse than just buying the robogear varan model and converting it, which is what I did for my warbuggies. But if you can only find the treads, they are great.
The grot turret from the battlewagon is perfect for grot tanks.