In order to determine if parts are available, first you need to determine what parts are needed. A lot of different things can cause an octave not to play. It may not even need parts. Often, when keys don't play, it is due to dirty contacts. When an entire octave doesn't play it can be a bad connection on a single wire. When multiple octaves don't play it can be a connector that is unplugged (sometimes only partially unplugged)

The CTK-496 keyboard has two circuit boards that make up the key switches. The lower board handles the lowest three octaves. It plugs into the upper board, which handles the upper two octaves. That board then plugs into the main board.

If you are missing the two lowest octaves, then I would check the connector that goes between the two boards under the keys. If it is partially unplugged, you could lose connection to those two octaves. If you do not find a bad conection, then inspect the board for cracks or corrosion on the circuit traces that go to the lower octaves.

Also, inspect for any spills or contaminants that may have fouled the contacts,and clean if necessary. You can rule out problems with the rubbing contact strips by swapping in one from a working area of the keyboard, to see if it bring steh keys back to life.

The part number for these contact boards is 1016 3667.
The part number for the rubber contacts strips is 1016 3902. Four are used on the keyboard.
The upper octave uses a different longer strip, part # 1016 3903

You did not mention where you are located. If you are in the USA Casio provides parts through the two distributors listed on this support web page. Or you can contact Casio parts directly at 1-800-223-2001. If you are not in the USA, you can visit http://world.casio.com/ to find the distributor for your country. I checked one of the USA distributors, PacParts, and they did list those parts but did not have them in stock. So there may be a wait time. Once you determine what, if any, parts are needed; you can contact one of the distributors to confirm availability and see how long it will take to get the parts.