There are a number of Sony rumor sites. Most are geared toward photography or other aspects of Sony business.
I can speculate why you don't see any noteworthy Sony Pro Video rumor sites.
Hardware rumors are a bit easier to control especially when component orders from outside suppliers are harder to discern. For example: if Apple orders new components for an iOS device or even a computer, leaks come from people with ties to the component manufacturers for example or otherwise spot financial reports.
Parts being manufactured internally obviously are much easier to control. Parts being purchased from suppliers, if they're not unique, can't truly be tied to a new product. For example, if Sony buys lots of APS-C sensors is it for still camera Alpha sales or a new video camera? There's no way to tie an order increase specifically to a new video camera.
Also keep in mind that the Pro camera market is very small compared to iDevices or even still cameras. Given the limited access and small scale of the component orders I suspect it's much easier to control NDAs and conceal development.