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It would seem that these are prone to mosaic virus, which will be in the soil permanently. There is no treatment for this, but you may be able to prolong the death by using fungicide. Just apply it as the package recommends.
https://www.gardeningknowhow.com/ornamental/flowers/cat-whiskers/growing-cat-whiskers.htm

It's hardy in USDA zones 9-12. In your zone 9, it should be able to withstand 20 to 25 degrees F. There is a good chance only the top growth froze and the roots are still alive. Wait till all danger of frost has passed, then cut it down to the ground. It should grow back.

https://www.gardeningknowhow.com/ornamental/flowers/cat-whiskers/growing-cat-whiskers.htm
https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

We don't make vendor recommendations but I did a search as you probably did and I didn't see anything in your country. An article I read echoed your frustration in finding seeds but the article said they are worth the search. Start by asking friends, neighbors, retail nurseries if they know of someone who has one and might let you take a cutting. Good luck!

You were looking in the correct spot, but it would not have had enough time to form seeds yet. The area will swell and turn brown when the seeds are ready.

The better option will be to grow the plant from cuttings. They propagate, asexually, very easily- Much like mint. You can take a piece of the tip of a plant (when it isn't flowering) or dig up a section with roots already on it. (This is what I'd do).

https://www.gardeningknowhow.com/ornamental/flowers/cat-whiskers/growing-cat-whiskers.htm

It is considered a tropical plant, so you will need to overwinter it indoors in a sunny window. If it has gone to seed, you can collect the seeds to plant next spring, or you can take cuttings and start a new plant indoors. Here is more on its care:

https://www.gardeningknowhow.com/ornamental/flowers/cat-whiskers/growing-cat-whiskers.htm