In eurorack modular systems the modules are mounted to the rails. The length of these rails is typically standardized to HP widths (1 HP = 5.04mm). Note that eurorack rails typically use M3 screws while the industry standard in Germany is M2.5.
Note that in addition to the (two) rails you will need:
screws that fit the nuts
square nuts that fit your rails (or something like this)
screws to screw into the side of your rails
BGT ("Baugruppenträger") on reichelt. For example this
Stefan Burbulla of Clicks & Clocks sells already assembled rails/frames. Have a look here. You have to order by mail.
(Note: "Rails" is ambigous. In an elctronic context it can also mean your supply voltages. In our case: ±12V. So if somebody says: connect the opamp to the rails, they don't mean you should connect them to the case, but to ±12V. If some signal hits the rails this means the signal's voltage is higher (or lower) than ±12V and gets clipped).