Like many other weapons, you can obtain the Destreza from two different ways. The first option, as always, is by buying it directly in the Market Console of your ships or through the Arsenal for 175 Platinum. This way you also obtain an extra weapon slot for your inventory and an Orokin Catalyst already installed. This is a simple and fast option, as long as you can afford it, of course.
The second option is to buy the Destreza Blueprint in the Market Console. Once bought, you will have to obtain the necessary resources to craft it in the Foundry of your ship. This is the slowest option, but is the one who practically all Tenno choose for being the most accessible.