i've gotten most of my plants from ebay at this point, and never had any big problems. just make sure you read the descriptions and reviews carefully, like with any ebay purchase! sometimes you can score free shipping too, it pays to look around a little. i've also gotten plants from plantedaquariumcentral, and they were great- nice healthy plants, and they sent extra.
re: special qt- i got a really annoying hydra infestation off of a plant from online once that somehow managed to survive my usual quarantine, so now i do a double dip on any plants i get, according to the advice of the all-knowing google. first, a 2 minute dip in a 1/5 hydrogen peroxide/water solution for general disinfecting purposes. bleach is more effective wrt bacteria but also a lot harder on plants, so i don't use it. second, a 3-hour soak in a solution of 3 tbsp alum/gallon of water. this kills off your snail eggs, bugs, and hopefully planarians and hydras. all this is probably a bit overkill, but i haven't lost any plants doing it and also haven't got any other hitchhikers so it seems to be working. :p if you don't mind hitchhikers, which for the most part won't really bother something as big as a betta too much- fry and baby shrimp are more at risk- i'd say just do a normal qt 2-3 weeks.