If you are looking for debian or ubuntu packages, then please see here.
For other systems we don't currently maintain official packages so you'll need to either build your own or contact a distributor.
The Synology NAS packages can be obtained from here.
The source for Tvheadend is hosted on github at https://github.com/tvheadend/tvheadend .
For a tarball of the latest stable release please see here - quick link for 4.0+ - https://github.com/tvheadend/tvheadend/releases (note that stable versions are ones with even middle number - .0, .2, .4, .6, .8).