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Does anyone know a cheap and easy way to convert grot tanks?

I think toy tanks would be a good starting place.

The tricky part is the tracks. You can build them yourself (check out Ian Wyatt's YouTube channel for some outstanding examples) but I took the easy way out and ordered some track pieces from Ramshackle Games. I went with the Vole Light Tank Tracks, but they have several sets that look like they'd work just fine.

The rest of the tank is easier - the hull is basically a plasticard box, then slap on bits, junk and rivets til it looks sufficiently Orky. For the turret, I'd look around for something roughly the right shape (like a large bottlecap, or even old toy parts) and add bits on from there. With Orks, it's hard to go wrong... if in doubt, MOAR BITZ!

are these cheap enough?
http://maxmini.eu/miniatures/goblin-junkyard-roller-miniature
http://maxmini.eu/miniatures/goblin-scrap-tankette-miniature
http://maxmini.eu/miniatures/goblin-scrap-tankette-mk2-miniature

see also here for alt turrets:
https://bitsofwar.com/12-vehicles-vehicle-bits

Like Wasteland said - Tracks are going to be the big challenge, especially if you want to make multiples. Are you at all familiar with casting? The old ork buggies had tracks that are very easy to make molds for and should be about the size you need. I think you can buy casting kits for $30.00 and they'll go a long way in reproducing some of those hard to find pieces.

Depends on what you mean by cheap.

a local player used these as the base for his convertions: https://www.games-workshop.com/en-NL/Kataphron-Battle-Servitors-Breachers and they look great

Nothing stops you making Grot Armoured Cars instead - wheels are usually a lot easier to find/convert than tracks.