X10 closed their factory in China early last year. I heard it was a fire, but in any case most products are not currently being produced. Once the stock is depleted, that's it for now. The only thing that won't show up as discontinued is probably the stuff they are just rebranding from OEM. They are pushing the cameras right now, but they were trying to sell the Android tablet and their whole house audio solution a few months back. As far as I know, they are trying to set up a new production facility for at least some of their products. I'm hoping they get back up and running as some of their stuff is still really useful. If they don't get set up again, the next best thing would be Insteon since it's partially compatible with X10 and can use the current stuff until the user decides to upgrade. The big down side is the price since Insteon is ~3-5x more than what we are used to paying for X10. Since the shortage started, that price gap has narrowed though.
In any case, you can find most X10 modules on ebay, Smarthome, etc. At this time, it appears that MS16A's are going for ~$16 shipped on ebay. FWIW, that's about what they used to go for before the shortage started.