Ah, I remember seeing those things >.> I've never dealt with them, but I remember thinking about getting one of the fake ones.. One of the reviewers on it comments on how they had marimo balls, but for some reason, in their betta tanks, the moss balls ended up withering up and dying. So they opted for the fake ones, and it worked great. To me the main problem is that after a while, you have to replace them every 2 months or somewhere around there. So technically you'd be spending more money in the long run, but at the same time with the actual Marimo, you'll be dealing with a living thing, which might end up dying at some point anyway.
Otherwise, they're very durable plants ^.^ They're actually algae, so you've got a huge ball of algae that keeps your tank clean from.. well, clean from algae, but it's really more or less containing it in one specific area if you think about it xD There's really not much care required for marimo. A bit of low light is needed, 'cause it is a plant, after all. Just keep in mind that it's not a matter of watts, but kelvin.. Not to strong of a bulb, either, 'cause you might just kill your plant from over-lighting it, and make sure your water temperature isn't too warm, either (from what I recall, it will do fine overall with temperatures a betta thrives in). You might also want to turn your marimo ball over every once in a while to prevent it from browning on one side. Otherwise, it's not necessary to have plant supplements, although if you were to split the ball, you might want to use some kind of supplement to help encourage growth.
If you want to keep its nice round shape, you should roll it around every once in a while. You're also gonna want to give your marimo a "bath" every once in a while to get rid of excess debree. All you have to do is squeeze out excess water it's soaked up (not too hard or you'll squish it!), put it under running water (not too warm) and make sure anything sticking to the marimo is washed off, and put it back in the tank ^.^ Sometimes it'll float because it doesn't have any excess water in it, but you can just give it another squeeze in the tank to let it sink. It'll eventually sink on its own, so it doesn't really matter... >.>
If you'd like to attatch it to something, you can rip it open and tie it to a piece of rock or drift wood and it'll eventually grow over it.
Also, just like any other plant the marimo helps soak up things like ammonia, and I want to say other stuff, but I don't recall at the moment.. I'm gonna have to look that up later.. >.>..... but yeah, there's rarely any downside to having a marimo ball because they're so beneficial to your fish in many ways, yet they're so low maintenance, they may as well be indestructible in comparison to other plants ^.^
No prob ^.^ Um, from what I recall, it's the plants way of photosynthesizing. They sink when they're done getting enough light, or food, and "during the day" or when they need light/food, they usually start floating. There's a kind of jelly fish that actually feeds off of light in a particular lake and they float and follow the light during the day, and when it's night, they sink back down. Same thing, pretty much.. I just thought that was cool xD
I really couldn't tell you where to get them.. The Petsmart around where I live actually just started selling them recently. You should definitely ask around, especially on the marimo threads on this page and in the 'lounge' section.. I'm sure there've been other people on there that've bought them online. From what I remember, I believe aquabid.com sometimes has some really good deals on marimo's, if anything.
They have them at my Petco, but they rarely look very good, they are usually more brown than green, and they are $9.99. I have a local fish shop that sells them for $8.99, so not much better, and they are smaller but at least they are nice and bright green :) Maybe try searching the YellowPages for a local place? Oh and don't let names deter you! The one near me is called "The Reef" so I never went in for the longest time, assuming they only focused on saltwater aquariums but turns out they have a TON of freshwater stuff too.