Need some Heavy Plate ( Carbon Steel )
1 off 630mm X 190mm x 18mm
2 off 150mm x 200mm x 18mm
Plate thickness needs to be about 18-24mm ? Carbon Steel
your best bet would be 20mm structural steel - its a stock size at most places
have you got a grade in mind ? S275 JR ?
we use this lot at work for shaped / cut to size plates: http://www.forthsteel.co.uk/
but if you just want rectangles then any decent steel stock holder should be able to do it for you