The rocker cover gaskets were, as said, not replaceable. The recommended replacements are made by a company called Thermotec. Can't see they've got any on eBay at the mo.
As for the rest.....
Inlet manifold gasket - 0349 G1 - marked as NFP on Servicebox
Water temp sensor housing - 1340 33 - still available from Peugeot, £2.78 Inc VAT
Injector o rings. Set of 2 - 1981 31 - still available from Peugeot but bloody expensive at £22.51 Inc VAT for a pair.
Get yourself onto Servicebox. Loads of info there....you can pick up the part numbers and use those to search the rest of the net.
Rallye .... On going OEM spec resto
GTi6 .... Daily driver
Ph2 Xsara VTS