Thanks for the suggestions. I only hear the noise occasionally. Is that still likely due to excessive valve clearance? I suppose there's only one way to know for sure. The car has done about 280,000 km so there's a good chance they do need adjusting.Back in 1991, I had a 240SX which used a KA24DE. I had to make some shim adjustments, so I was able to get several shim sizes from a Nissan dealer. I would assume that the KA24DE shims will fit a GA15DE. You might also check out high perf companies like Enjuku Racing and VividRacing.
Another option is to use one of your old shims as a sample for fitment purposes with shims for motorcycles.