I have an item of clothing with a tag labeled "Marilyn Monroe".. But in cannot find anywhere that a?

Perhaps eBay or Amazon. I've never heard of a clothing brand named "Marilyn Monroe' , but I do know that Guess and other clothing brands have made jeans etc with Marilyn Monroe featured on them. A friend of mine had a pair of Guess jeans with Marilyn Monroe rhinestoned onto the back pocket....

Also I just recently bought a shirt with Marilyn Monroe on it and although a secondary company did make the apparel, Marilyn Monroe's signature was also used on the inner tag.

Hope this helps :)