where can i buy original apple silicon iphone x case?

If you can’t find one for the X then look at one’s for the XS, basically the same size.

iPhone XS Silicone Case - Hibiscus

https://store.apple.com/uk/xc/product/MUJT2ZM/A

If you look under the Q & A section it mentions a slight gap around the camera of about 0.5mm.

The Apple Store is where I bought my red one.

You can also find them on sites or in the stores of others retailers like BestBuy, etc.