On the strip there are really four main shopping malls. There are the Forum shops at Caesar's, the Canal shops at the Venetian, Miracle Mile shops at Planet Hollywood and then the Fashion Show mall which is across from the Wynn and next to Treasure Island. You will know your there because thernis a giant flying saucer looking object in front.
To me, the Canal shops are the most upscale, but the Fashion Show mall is by far the largest mall of the ones I mentioned. If you can't find what you need here then most likely it won't be easy to find elsewhere unless it's something like a rare watch or ultra high end items which I jus admit, Canal shops and Forum shops may have Fashion Show edged slightly in high fashion designer items.
Overall though, this is a nice, spacious mall and it's large enough to easily get turned around or lost inside. They have a nice, open food court too which could benefit from a few more restaurant selections but it's still great for a quick bite on the run.
Enjoy!