any v8 supplier (landrover/tvr/rover p6) should be able to help.
The people who support the p6 directly are http://www.winsintltd.co.uk
and jrwadhams. (worth supporting as without specialist suppliers we'd never get those rare p6 only parts!)
alternatively you could go to ebay, rimmerbros, or land rover direct.
Your car should have the tin gaskets which you should stick to unless you want to change the compression ratio, although if you have a choice of valley gaskets i'd take the opportunity to go for the composite gasket.