Hi,
The ux31a has its own proprietary 18 pin connector. Only 2 hard disks were produced for this model, Sandisk U100 and ADATA XM11. Both can be found on eBay (but are quite expensive) or aliexpress (but I'm concerned about the genuinity and reliability).
A good alternative is to buy an adapter, from m.2 to this 18-pin thing, so that you can buy a much cheaper m.2 ssd on your own. This is available from Chinese vendors on both amazon and eBay. I'm currently waiting for my own to arrive, but amazon reviews are pretty good.
Hope this helps!