I have D'Link DIR 819, and want to flash Owrt in that, how I can proceed, because official there is no any release for it, can I flash other D'link variant like DIR 850 or some others ?
I also have TP link WR850N and for that also I think it is not available, kindly help me, to do so.
No, you can't.
WR850N v1 v2 v4 are however supported
These are embedded devices, which each need their own bespoke image exactly tailored to their (hardware- and software) requirements. There are no 'generic' images that will work on multiple different routers, either your exact model (down to its hardware revision) is supported - or it isn't (if it isn't, you may be able to propose a pull request adding support for it, but the necessary down-to-the-hardware development would be all yours, as you're the only one with the hardware on the desk).