Firstly you need to measure the valve clearances. When you have determined which (if any) shims need replacing you will need to know what size shim is already fitted so you can work out the sizes you need.
You will need to remove the valve cover but the rest can all be done without removing the camshafts.
Shims can be found on fleabay but generally they are large sizes for which there is very little call as usually shim changes are for smaller ones and they are not cheap. I believe there is a place in OZ making them still but again not cheap.
However I do have quite a large stock of shims here.
As far as valve gaskets are concerned if you remove the cover carefully you may be able to reuse the existing gasket. Even a rip does not render them useless. I have successfully reused gaskets in 4 pieces by using liquid gasket on both sides and piecing back together. Personally I have had no trouble with aftermarket gaskets but I always use liquid gasket. Some members have had troubles with Athena gaskets.