The Fracture Case is a prime CS2 item which has a chance of being offered to players with prime status once per week as part of their weekly care package.
The weekly care package contains four items, of which one or more random weapon cases are selected from the prime case pool and/or rare case pool to be offered to the player. Only two out of the four items offered can be redeemed.
Since the Fracture Case belongs to the prime case drop pool it has much higher odds of being offered to prime players on rankup than rare case drops such as the original CS:GO Weapon Case.
Alternatively, it can be obtained by trading with other players, or purchasing from the Steam Community Marketplace and some third-party marketplaces.