You can TRY feeding them egg yolk like reechardd said but... you really should have started live vinegar eels or microworm cultures before you even introduced the pair. Sorry. You'll probably have some survive but they do substantially better when fed live foods from the beginning. I say try to get your hands on an already started culture asap.
I'm not sure if anybodies noticed... but apple snails poop A LOT. So they don't improve your water quality,
they actually decrease it faster, which is counter productive and means you have to do more water changes. So adding one is not something I'd ever do.
I would
highly recommend cycling your growout tank before hand next time. It will make water maintenance easier. So for future reference add a sponge filter and cycle the tank. An unfiltered growout tank will result in stunted bettas and a higher death rate.
Here are some great articles I would read thoroughly.
http://www.ultimatebettas.com/index.php?showtopic=3686
http://bettysplendens.com/articles/catview.imp?catid=855