I am trying to find a cartoon of man looking for keys under a street lamp.
I know I have seen one in the past, but I cannot find it. Can you help
direct me to one?

This description sounds a bit too general.  Do you recall any other
details of the cartoon or perhaps any details of where you saw it?

Is the cartoon related to this joke?:

"... the old joke about a drunk who was looking for his keys under a
street lamp. He had dropped the keys elsewhere, but the light was
better under the streetlight..."

http://www.atr.org/press/editorials/tas/tas0897.html

Yes, the cartoon is related to the joke about the drunk looking for
his keys. When asked why he was looking for them away from where they
were lost, he said the light was better under the street lamp.

I haven't been able to locate the cartoon.  I've found many references
and versions of the joke/story.  I've tried clicking on many of the
websites which make reference to the joke/story in hopes of finding a
cartoon, but no luck.  There are far too many references to click on
all of them.  Here are the results of the web search that proved to be
most helpful.  Maybe you can find the cartoon:

://www.google.com/search?hl=en&lr=&q=drunk+keys+street+lamp