I recently had my 1959 Mark 1 3.4 Litre XK engine worked on. The valve cover gaskets obtained locally (U.S.) are poor quality and leak oil. Can someone tell me where to get high quality gaskets?

 

Are you sure it is the quality of the gaskets or is it that the top of the head and the cam cover are just not a good fit or cleaned properly. SNG Barrett supply the correct gaskets but whether they are a better quality is to be seen. When I fit my cam cover gasket I always add a small seal of instant gasket to both sides to take out any imperfections that there might be between the two sides as the modern paper gasket is in my opinion very thin compared to the old cork gaskets you used to get.
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A common cause of cam leaks is a warped head, check the mating surface of the head with a straight edge.

Warped heads get the deck planed, but the machinist fails to straighten the entire head first before planing, leaving the top part still warped.

 

When I had the head off my 3.8 last winter I was amazed at how rough the gasket mating surfaces were. They hadnt been gouged or poorly dealt with, but they were not smooth. It reminded me just a bit of rough sawn timber, the saw marks plainly visible. That issue compounded by the cam covers being cast aluminum made me not want to risk cracking them by over tightening so I used a sparing coat of gasket sealer on both sides as Rob suggests and have not seen any leaks so far. I used standard paper gaskets and black rtv. My 2 cents.

 

As above I would work out where the leak is from and if its a mating surface then a new gasket and smear of RTV will probably fix it.

Just remember XK engines never leak oil, they just sweat power!