Most metric fasteners are always sold separately, so if you need ‘nuts’ and ‘bolts’ then you may need to buy two items.
The reason for this is that the metric standards (especially the threads) allow for multiple types of bolt / screw to be used with multiple types of nut (and vice-versa).

For example: The metric thread standard (known as ISO965) means that any type of standard M’8′ threaded bolt / machine screw, should fit all the equivalent standard M’8′ nuts.
There are dozens of different types of nuts and bolts, so to have every combination supplied with every other combination would be almost impossible.

Also, both the nuts, and the bolts are often used without each other. A nut can be used to fasten items to a threaded shaft that might already be fitted to an appliance or structure.
Bolts and Machine Screws are also very frequently used to fasten items to pre-threaded holes that might be present in appliances or structures.

​The ‘thread’ will be identical across all the types, so for some purposes the type of nut / bolt can be interchangeable, but for most applications there is a specifically ideal type to use.

​Because of all of the above, supplying nuts and bolts together can actually be very wasteful. The nuts may not be required, the bolts may not be required, or either could be an inappropriate type for the application they’re intended for.