Where can I buy one and how much it will cost me?
My laptop is Asus Zenbook UX31E

IIRC that is an OEM SSD only sold to Asus. SanDisk does not officially sell OEM SKU SSD in retail. You may be able to find it on ebay or something like that but I doubt it would be supported by SanDisk as it is an OEM SSD. That said you could also check with Asus and see if they sell it separately.