What NoisyAssassin said. I've found my Daemon Assault Engines on ebay.co.uk, but these can obviously be pretty much anything with tracks, skulls (important!) and spiky bitz.
For the Greater Brass Scorpion it's finding the Forumware, or converting something like this
http://heresyminiatures.com/shop/index. ... ucts_id=71 or maybe something from Dystopian Wars like the Covenant of Antarctica which haves some cyber-crab-thingies of roughly the right size.
http://shop.spartangames.co.uk/SearchRe ... p?Cat=1953 This from exodus wars might work as well:
https://hlsmodels.com/collections/exodu ... ng-machineThe Tomb Scorpion model to Warhammer Fantasy (RIP) would be the right size, and would probably require a minimum of conversion.
https://www.google.se/search?q=tomb+sco ... 93&bih=798Shapeways have Scorpions of different sizes, could probably work as both as Assault Engines and Greater Brass Scorpions.
https://www.shapeways.com/marketplace/? ... e-productsThen there are these Necron things from Forgeworld. With some adjustment they would probably work very well indeed, and you get 3 for £42.
https://www.forgeworld.co.uk/en-SE/Necr ... canthritesMight end up using this option myself. Can't see myself using more than three of these anyway.
There's also two alternatives from Forge World 30K Mechanicum:
https://www.forgeworld.co.uk/en-SE/Mech ... e-Automata and
https://www.forgeworld.co.uk/en-SE/Mech ... Ursarax-FW