The SR300, R200 and ZR300 RealSense Developer Kit cameras are being retired, so there will be no more stock of them once they are sold out, unfortunately. They are being replaced by the new D415 and D435 cameras, which are due to be available to pre-order on the Click store from Monday 8 December and be released for sale on January 8.
The SR300 will continue to be available without an outer casing in circuit board form as a Module Kit.
If you need an SR300 USB camera, you should still be able to purchase a Razer Stargazer or Creative BlasterX Senz3D camera. These cameras by Razer and Creative are fully compatible for use with the RealSense SDK as an SR300 camera.
