This is one of those cases where e-bay will be better than the ME suppliers, just put "4mm steel sheet" into e-bay and you will get the choice of cut to size, buying a nominal size eg 200×200 or find an offcut from a laser cutters or steel fabricators.
If all your ebay results are in the US are you using e-bay .com, try using e-bay.co.uk instead
Edited By JasonB on 12/03/2017 07:52:44