Where can I find Channel Presets?

[Hidan]

Those ".fst" files, which are the instruments used by FL Studio, where can I find more to download?

[Hidan]

These .fst files can be obtained from buying them only?

[Zephyr]

Well, the fst files are specific to each individual instrument, and the instruments that use them are FL-Native plugins, so it depends which plugin you're looking for presets for. If you want them for a specific plugin (it must also have a VST version though) you can search it here: http://www.kvraudio.com/get.php then check for available soundbank downloads.