Besides compression tights, hip padding, and waist cinchers, the most important piece of shapewear for kigs depicting female characters are silicone breast forms. Basic, circular ones can often be purchased together with a hadatai, but for the best results one may want to consider 3rd party options from specialized manufacturers like !vita.
Unlike real breasts these aren't defined by cup size, but rather by the combined weight of a pair plus their individual length, width, and height. Since this makes for many different shapes and densities, it is essential when talking about silicone breast forms to mention these four variables. As a rule of thumb; lower density forms will feel looser and have more jiggle, whereas higher density and most heavy breast forms feel plump and don't have as much give. Neither is inherently better or a mark of quality; it comes down to preference.
For reference, here are some popular shapes of silicone breast forms with various applications depending on body type. Note that the images below are all from !vita, and their measurements may deviate by up to half a centimeter.
Round or circular forms are the ones most commonly used by chinese kigs. Sold by hadatai manufacturers these sometimes come with velcro on the back to attach them to a vest inside the hadatai, depending on the manufacturer's offerings.
They work best to give small and slim body frames a bust with a lot of protrusion without getting too wide or long. However for taller physiques the lack of a smooth transition upwards often makes them look unnatural unless hidden under clothing.
Teardrop forms are something like the allrounder. They're not especially wide, and give a better transition to the upper chest than the circular blobs. Recommended if you're not sure what to pick, however in some cases they may have too much height/protrusion for a modest look, and may not be wide enough for bulky physiques.
Larger teardrops for when 2000g or less just isn't cutting it. It's generally recommended to get a pair of the standard teardrops before to jumping into the bigger sizes. Really big forms above 2300g don't increase in size by a lot but become much denser.
Concave back forms are unique in multiple ways. They have a very low density, are very wide and long at a comparatively low height, and have the smoothest transitions. This makesthem a great pick for tall body frames with long and slightly wider upper bodies, since they give a more modest look with less protrusion than that of teardrops.
Triangle ones are the most niche forms on this list. They are generally only recommended for wide but short body frames, providing less length and more width at similar levels of height/protrusion as the standard teardrops. They're good in case the standard teardrops either aren't wide enough to cover your chest or are too long, but you don't want to go bustier. If you don't have a short physique with a very wide upper body, chances are these will poke your armpits.
Note that for the circular and teardrop shaped forms there are sizes above 2000g, however these usually do not have a conveniently grouped listing like the ones from 500g to 2000g, and will have to be specifically searched for.