which hard disk I can buy for my MacBook Pro?

Sadly, you're between a rock and a hard place right now. You see Apple currently is the only supplier of the SSD that will work in your system (replacing your current SSD drive). But there is light at the end of the tunnel here! OWC has a tease on their web site: OWC Upgrades for: Late 2013-Current MacBook Pro Retina So the long wait is nearly over!

Now if you can't wait there are a few things you can do in the meantime. First I do recommend you do a bit of house cleaning at there is often lots of hidden junk that can take up space neanlessly. I would recommend using a tool like this: Disk Doctor.

You should try to free up about 1/3 of the disk on a smaller drive (128 GB ) and 1/4 on a larger drive (1 TB) so the system has some free space for virtual RAM. What you'll need to do here is off load as much as you can to an external drive. I would get something like this: Samsung Portable SSD T1. While this offers great alternative storage it does force you to carry something more. The next option is to use your SD slot with a second SSD unit. While not as large, it maybe enough for now. Here's one unit: Transcend JetDrive Lite.