Mini mSATA SSD. I don't think it's user accessible, you'd have to open up the keyboard plate to get to it, if I recall correctly. It doesn't seem hard at all, but it probably voids your warranty if there's any telltale signs you've been in there.
You can add ssd to standart disk place, and standart hdd you can put to optical bay with hdd caddy. But this hdd will always spin, never sleep. Like my.
MyDigitalSSD are pretty much the only company that makes a decent version of these, yeah. Probably your best bet, there are some others but these ones are quite popular.