Yamaha refers to SFF1 as SFF in their ads and owner's manuals, and refers to SFF2 as SFF-GE (GE is short for Guitar Edition, since as I understand it the differences between SFF1 and SFF2 basically have to do with the way guitar voices and guitar chords are coded in the style files). Both formats use the same file extensions, so you normally can't tell them apart just by looking at the file names, but there's a MIDI text message inside them that identifies them as either SFF1 or SFF2. Note also that there are two basic formats of Yamaha-compatible style files- SFF1 and SFF2 (SFF is short for Style File Format). There are free third-party utilities you can use for doing that, available from the PSR Tutorial and jososoft websites. Note that unless a particular style file was specifically created or customized for your given model, you will usually need to re-voice it for your model, which means to view the voices (that is, the specific Bank Select and Program Change messages) used by the style and change them as needed so that (1) they select the best-sounding voices that your instrument has, and (2) they do not try to select voices that your instrument does not have. For instance, check the PSR Tutorial website and forum, as well as the jososoft website.

I believe there are third-party companies that create and sell Yamaha-compatible style files, and you can find a huge number of free Yamaha-compatible style files online. I also check the alternate and related model names (YPT-400, YPT-410, YPT-420, PSR-EW400, PSR-EW410). What I do is enter the oldest model in the line (PSR-E403) and see what comes up, then change to the next-oldest model (PSR-E413) to check again, and keep doing that until I've checked up through whichever model I have.

Yamaha sells style files on their YamahaMusicSoft website, although the number of styles which show up as being compatible with your instrument can vary considerably depending on how new your model is.

How to create yamaha style files