I use sections of dried banana leaves in my betta's tanks, they float at the surface and my bettas love them. You can always bet that if they haven't built a bubble nest under them in the first hour after you put it in the tank, there will be one there by the time you wake up and check on them the next morning. These are natural alternatives though, so they do rot - I don't leave a leaf section in the tank for more than a week at most, but they are one of the closest things you will be able to get to a bettas 'natural' environment. Be careful though, if you are going to try this, make sure that there are no pesticides or organo-phosphates of any sort used on the trees the leaves came from, as these will surely kill your betta as the chemicals seep into the tank water. Also note that banana leaves have tannins in them, which will slightly stain the water a bit of a brown color, like tea. This is okay, if perhaps you may find it a bit unsightly, because the tannins are actually good for your bettas and help to prevent infection.
Styrofoam vs polystyrene. <-- this is a common misnomer. There is actually no difference. Styrofoam is really just the largest, and most commercially recognized brand name of expanded foam polystyrene packaging. The only time you will see a difference is if you are using extruded insulation styrofoam, but honestly, unless you are in the construction, engineering or architectural fields, I'm not sure where you're really going to run across that. Any of the styrofoam that you are going to find in your disposable coffee cups or packaging peanuts is expanded polystyrene foam.