Shopping app

I'm looking for help to develop a shopping app that is available on both iOS and Android platforms. I do not have a complete design yet, however I have a basic concept that will need to be refined. My app needs basic functionality with few features and no complex features or integrations. The development team should be able to handle both the design and the coding. I need the quality to be high and to create an immersive and enjoyable user experience. They should deliver the finished product within a realistic timeframe and budget.