You will find most of his best known works available as iPhone cases, particularly on Amazon who host a number of third party sellers. Always check reviews in detail if you are ordering from a company that you have not used before, but mobile phone cases tend to be fairly reliable, as each one must fit the particular specification of your phone. The only real concern, therefore, is that the printed photograph is sufficiently faithful to the original Dali painting and in good enough resolution that it is clear and bright. There are a number of different places to buy them from, and so it is worth doing some research if you have not done business with these companies before.
It is the younger generation who are particularly keen on customising their iPhones or Androids, and these cases are normally for sale at highly accessible prices in order to take that into account. You should not really be paying more than about $20 or £15 for these, unless there is something particularly unique about the product. Most of Dali's famous paintings will be offered as part of the retailer's product listings, but it might be worth trying to customise them where possible in order to better suit your own tastes. For example, you might need to move the original artwork around so that the key parts of it are featured, as the cropping might not work effectively otherwise.
Another thing to consider is that Salvador Dali iPhone cases can also make brilliant presents for others, be it friends or family. All you would need is the model type of the recipient's phone, to ensure that it will fit correctly. Each phone comes in different shapes and sizes, and you also have to account for the positions of charge connections and camera openings, otherwise the phone's services will not be reachable. Another benefit is that it can make it easier to identify your own phone, when a number of people in your household have mobiles from the same company and perhaps put them together on occasion.
Those who really love this artist will need to make sure that the images used on the covers are genuinely from the artist's career, as some retailers will use artworks inspired by Dali instead. These will tend to lack the magic of the great Spaniard, but only those knowledgeable about his career will be able to spot the difference relatively easily. Some companies that do not hold the necessary copyright permissions will go down this route, but ultimately the quality will not be as good as ordering a direct reproduction of Dali's work, even though that might be slightly more expensive as a result. The best option is always research, across different stores and through many reviews from previous customers to give yourself the best chance of making the right decision. Foreign stores may also incur additional costs and so be carefuly about where your chosen case is arriving from, sometimes that is not as clear as it should be.