Damascus steel

[timj]

there's no reason why a 4-5 mm sheet couldn't be milled or ground down to 2-3 mm. Yes you're' wasting a bit of material, but it's going to be hard to find any thinner than that

 

[jont300]

you may be able to get some some mokume gane in thick enough sheets similar Damascus steel but gold,silver,copper,brass. gold or silver mokume gane is expensive but copper and brass should be far less, i did see some 2mm thick sheets a while ago but i cannot remember where