Is it right that there is only a very limited number of plugins that will do side chaining. Is it not possible to side chain any third party plugs or do they have to have specific side chaining ability built into them? Does SONAR not have a mechanism for giving a side-chain input to any plug you chose?

There are quite a few plug-ins with side chain support. It must be built into the plug-in. SONAR cannot create a side chain if the plug-in lacks support for the feature.

Side chaining is supported by a couple of Sonar's bundled plugins, most notably the Sonitus compressor/gate. It is not supported officially under the VST2 spec as far as I know, but it is under VST3. So I believe most relevant VST3 plugs support it.

In addition to the 2 Sonitus plug-ins, the PC expander/gate, both S-Type compressors and the CA-2A support side chain.

42?

> Does SONAR not have a mechanism for giving a side-chain input to any plug you chose?
 
If the plugin doesn't support it you are out of luck (no DAW will sidechain by magic sadly).
There are plenty of plugins that do support it, and plenty that don't. For those that you think should support it badger the developer (offer him beer or something).
 
Cheers :)