Where to get Mild Steel Plate?

[Chaz]

I'd go with steel if could work with it. However I'd only use it if could do it correctly which means will need heat treating and stress reliefing then grinding in critical places.
If you can't do this then you may find Ground tooling plate will be more accurate as it's already been heat treated and ground flat.

Plug welding will allow thinner plate to be used but all the above still applies and will require more work because all that heat will warp and put stresses into the whole assembly. If those are not released carefully then they will release eventually which will make for bad day when you do realise.!!

Yep, this I cant do at home, so looks to me a limited option for me then.