If you can afford to spend a little more, consider purchasing this Marni Cross Body Bag. While the price is very reasonable, this item is an easy wardrobe update.
If you're willing to splurge, then what you're looking for is this Marni Crossbody. It's more expensive but worth your money. Another product we're drooling over is this Marni Crossbody Bag. It's expensive but you get what you pay for.