every place i mentioned is somewhere i went to, and liked. The way I find them is usually just getting in the mood for something and looking up "[blank] food columbia", or sometimes I just drive past a place and go "that looks interesting, i should go try it sometime". If you want a staggering amount of really delicious and varied asian restaurants I highly recommend literally just driving down rt 40, theres a toooon of places tucked into random strip malls or shopping centers.

Also the main reason these places dont do marketing is because they dont make that much money and the owners often dont speak english very well, but if you look for them they all have some type of website at least.