You obviously bought, whether you knew it or not, the OEM version of this SSD. In a plain brown box with a sticker in it, and nothing else, right?
The retail version comes in the typical blue Intel box, with an adapter plate for mounting in 3.5" HDD bays, a SATA cable, a molex to SATA power adapter cable, and eight screws.
The OEM version is usually cheaper, but that can vary. They are really meant for companies building PCs, so no mounting hardware is supplied. That's nothing new, been done for years.
Complain to your retailer if they did not inform you that you were buying the OEM version of the 520. If you received a retail version without hardware, return it for another.
I have two sets of screws:
1- is to attach the SSD to a 2.5" to 3.5" bracket (spec: M3 X0.5 5MM,)
2- is to attach the bracket to the computer chasis (specs: UNC 6-32,5MM,)
I hope this helps