I've found M-Machine Metals pretty good for that kind of stuff, they often have big slices of CI or steel at reasonable prices, it's pot-luck what's in their Ebay store, but there's usually something suitable..
M-Machine Ebay store
..or get contact details from their website and call them
M-Machine Metals Darlington
No connection with this company, but have usually found something useable in their listings and had reliable service.
.. if you don't mind CI their GR17 is very 'clean' and nice to machine..
Edited By DiogenesII on 19/10/2022 17:54:44