Where to get Mild Steel Plate?

[magicniner]

Not sure what Plug welding is but I had considered welding up a few layers of say 10mm to make up what I need. Thanks. Certainly an option.

Chaz,
Plug welding is welding through a hole and "plugging it".
Drill a pattern of holes diameter 2x thickness in one plate, clamp it to a second plate and weld through the holes in the first to the second plate and the hole walls, finishing by filling the holes then grinding flush.
Add further plates in the same way until the desired thickness is achieved, you could optionally weld around the outside,

- Nick