the reason you find it hard to find sights that premote sub-tropical fish is because it is a relative poor area of most people , most people just assume that any fish bought from the lfs is either tropical or cold water , they do not even think some of them are actually sub-tropical or temperate species, this is not helped by the shops who do not know either . unfortunatly things will most likely stay this way as most temperate and sub-tropical species will tolerate tropical temps year round , so no one is any the wiser , however snakeheads are no normal fish , any suffer really bad without a period of cooling over winter , so really they are best kept on the own in a small group in a species tank , as rainbows are peacefull you could mix in a comunity tank as long as those fish have the same requirments of water temp , to find this out you need to studdy native fish from sub-tropical india burma and the hymalaya mountain range and set up a type of bio-tope setting for them.
if you want to make life easier for your self but still want a snakehead , then choose a tropical species such as bankanensis , or a tropical varient of gachua , however i would still recomend you allways keep a snakehead in a species tank , as in the long run it rarley works out
cheers col