Re: I've had a good look for it, but I can't find it.
The suggestion is that even though the thorough search took place at some point in the past (I've had a good look), the item is still missing so the person is probably still looking for it even if they are not making much effort to do so. The fact that it is still missing means that we can say "I can't find it".
Also, using "I have had" is naturally followed by the present tense in a lot of contexts:
I have had a lovely day but now I am tired.
I have had a great holiday and I am ready to go back to work.
I have had a lot of great meals this week and I am ready to start my diet.
I would have used "couldn't" in your example had it started "I had a good look" (not I've had")
I had a good look for it this morning but I couldn't find it. I will have another look later.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.