If you want some really nice silk plants, but can't get to a petshop, check out Acumen Aquatics on facebook (or join www.fishlore.com
and PM Lea). The lady who runs it lives on mainland Australia (hello from there from me too, by the way!) and ships to Tassie. :) They are completely betta safe and look really nice, much better than most plants you'll see in petshops.
You probably won't be able to find a nerite - the only ones we can usually get in Australia are Mystery, Ramshorn and sometimes Malaysian Trumpet, none of which I would recommend. If you can get ghost shrimp, though, they might make some nifty tankmates. :)
Does Tasmania have a version of allclassifieds, like we have in NSW/ACT? If so, that's always a good place to pick up some cheap fish, tanks and accessories. :) Also, if you have a local aquarium society, I suggest joining. Again, cheap stuff, plus heaps of knowledge and experience. :)
P.S - Diablo: if someone is speaking in litres, there's a good chance they aren't in the USA, so they probably won't have a Petco. :)