Khmer Language, Fonts, Keyboard Layouts?

Can anyone tell me
where to get a keyboard layout for Mac OS?

This might be a source (you could write and ask about the status):

http://www.xenotypetech.com/osxCambodian.html

Or you could make one using this tool:

http://scripts.sil.org/cms/scripts/page.php?siteid=nrsi&itemid=ukelele

How can I set the language to Khmer if I don't see it
in the list?

If you mean System Preferences/International/Languages, then use the Edit button to add it. However, this will not really change anything on your machine, since no Khmer localization is available.

Finally, will TextEdit or Pages allow me to type in
Khmer characters and convert them to Unicode? I have
the Limon Unicode font installed, but don't know how
to use it.

TextEdit and Pages and similar OX apps produce Unicode by default. Could you provide the url for your font? I think Limon may not be Unicode. One source for Unicode fonts is here

http://www.travelphrases.info/gallery/Fonts_Khmer.html

Without a keyboard, you can input directly from the Character Palette. View = Unicode, Tab = Unicode Blocks, block = Khmer (or Tab = Unicode Table, 1780 and onward).

It may not be possible to display totally correct Khmer with the currently available fonts, which are designed for Windows. You will need to try and see how it works.

Let me know if you have more questions.

PS You can also work in OS9/Classic mode with tools like this:

http://www.linguistsoftware.com/lcam.htm#Macintosh