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These are very tolerant of hard pruning. This article will outline how and when to do this:

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/pruning-bougainvilleas.htm

You can propagate them with softwood cuttings. Here is how:

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/propagating-bougainvillea.htm

Bougainvilleas need direct sun, warmth and consistent moisture to thrive. Growing them indoors is always a challenge. I'm not clear whether yours was outdoors to start and it was brought in for winter or you had recently purchased it, and it has always been indoors. They typically are an outdoor plant and in cold regions are brought in for the winter. They normally go through flushes of bloom followed by a period of nonblooming. But indoors it is hard to give it enough light without supplemental grow lights. For the winter, you can let it go dormant and lose its leaves and just water enough to keep it alive till you can get it back outside in the spring.

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-winter-care.htm

These are beautiful, and one of my favorites. Yes, you certainly can propagate them from cuttings to fill in. This article explains it pretty well. I don't think you should need to buy more plants. You have a wealth of cuttings available here. It is easier though to propagate from the green, softwood cuttings rather than the woody ones.
https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/propagating-bougainvillea.htm

It probably didn't need repotting yet. They actually like to be a little potbound. And it's usually best to repot when they are between bloom cycles. I think it is just in a bit of shock. Also make sure you didn't go up more than one in pot size. Also check on your watering. Let it start to dry before watering again.

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-garden-care.htm

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-in-containers.htm

Unfortunately, your photos did not come through. The plant may be lacking light, or nutrients. It would be hard to say without seeing the state of the plant.

In order to get the plant back to its happiest state, it will be necessary to care for the plant as flawlessly as possible. This article will help you to know their requirements:

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-garden-care.htm