I bought this one recently. Its a little expensive, but its an official one made for xbox one by seagate, so decided to spend the extra money to get it as i had gift cards i needed to use anyway.

If you want one thats much cheaper that should work just as good, get this one:

Drive: https://www.amazon.com/Silicon-Power-Performance-Internal-SP256GBSS3A55S25/dp/B075RJS55D/ref=sr_1_3?ie=UTF8&qid=1548282759&sr=8-3&keywords=ssd+256

Case: https://www.amazon.com/Sabrent-Tool-free-Enclosure-Optimized-EC-UASP/dp/B00OJ3UJ2S/ref=sr_1_4?ie=UTF8&qid=1548283038&sr=8-4&keywords=ssd+enclosure