I think this write up i did a few months ago should do the trick, just read it carefully:
Now, I don't know if you plan on replacing the valve cover gaskets again, but there are some tips in that link which explain how to do it really well so that new gaskets last until the car dies.
as for gaskets, go to rockauto.com and do a search for:
"BECK/ARNLEY valve cover gasket for 1992 lexus ls400 "
It will show you many options, but the best one is the one that costs $36.79 - that one is for both sides, so you only need one package.
The other gasket you need is lexus part number 22271-50010 which is just a cardboard gasket for throttle body. Since your mechanic did a lousy job, I imagine he did not clean the throttle body, so you may as well do it. There will be a thick layer of carbon in there, which often creates problems with throttle and accelleration. Cleaning procedure is also explained at that link above.
So not too many gaskets, only those two.