You can buy it as a part of Mustang and The Channel for $69.98 USD
The DCS: P-51D Mustang offers both highly-detailed simulation and easy-to-play "game" mode options for both hardcore and casual gamers. When in simulation mode, this is the most authentic simulation of the P-51D Mustang that has ever been done for the PC. Enjoy both the thrill of flying this warbird and operating its various weapons against a variety of ground and airborne targets.
An interactive training system puts you in the cockpit with an instructor as he walks you step-by-step of learning to fly the Mustang.
A powerful yet easy-to-use mission editor allows you to create your own missions and campaigns. A one-click Mission Generator also allows you to instantly create battles as small or large as you wish.
Fly online with built-in server browser that supports up to 32 players in both head-to-head and cooperative gameplay. Fly online with other DCS aircraft like the Black Shark and the A-10C Warthog.
Free disk space is required: 700 Mb
Minimum system requirements (LOW graphics settings): OS 64-bit Windows 7/8/10; DirectX11; CPU: Intel Core i3 at 2.8 GHz or AMD FX; RAM: 8 GB (16 GB for heavy missions); Free hard disk space: 60 GB; Discrete video card NVIDIA GeForce GTX 760 / AMD R9 280X or better; requires internet activation.
Recommended system requirements (HIGH graphics settings): OS 64-bit Windows 8/10; DirectX11; CPU: Core i5+ at 3+ GHz or AMD FX / Ryzen; RAM: 16 GB (32 GB for heavy missions); Free hard disk space: 120 GB on Solid State Drive (SSD); Discrete video card NVIDIA GeForce GTX 1070 / AMD Radeon RX VEGA 56 with 8GB VRAM or better; Joystick; requires internet activation.
Recommended VR systems requirements (VR graphics settings): OS 64-bit Windows 8/10; DirectX11; CPU: Core i5+ at 3+ GHz or AMD FX / Ryzen; RAM: 16 GB (32 GB for heavy missions); Free hard disk space: 120 GB on Solid State Drive (SSD); Discrete video card NVIDIA GeForce GTX 1080 / AMD Radeon RX VEGA 64 or better; Joystick; requires internet activation.
This product can be downloaded and installed through the DCS World Module Manager. If you do not have DCS World installed, you can download the DCS World web-installer by clicking this link. If the product description does not have information about which DCS World version you should install, you only need to have the "Stable" version DCS World installed on your PC.