Hey Taka, welcome to the forums. Activated charcoal for potting mixes is sold by companies like Scott's and you can typically find it next to sand and gravel and other soil additives in the gardening section. It's used to absorb and neutralize chemical impurities in the soil.
Dried sphagnum is kind of ugly, but if you keep it wet for a few months it will grow into beautiful, lush green live sphagnum (or, if you're lucky, it'll grow into red sphagnum, which is particularly striking.) You can also get some strands of live sphagnum from a forum member here (I'd be glad to contribute some.) Just a few two-inch strands of the live stuff can fill a terrarium in a year or so, judging by how fast mine grows.
Don't use clover because you'll never, ever be able to get rid of it. If you want a weedy ground cover for your terrarium, get a bladderwort or a pygmy sundew. Or someone could supply you with some D. capensis seed - D. capensis, the cape sundew, is a very prodigous weed of a CP that grows about 6 inches tall; the seed is always in great abundance here on the forums. I think that's a D. capensis in LleopardGgecko's avatar photo there.
So far as other plants go, you can get a lot of stuff by sending SASEs (self-addressed stamped envelopes) to folks on the forums. I could give you a bunch of sundew seeds, both indoor and outdoor species. Pygmy sundews often come up on the trading forum as well, because every year each one produces dozens of little baby sundews. Bladderworts are also available readily on the forums, and although you don't see much carnivorous action - bladderworts trap things in the dirt - they do put up big shows of flowers, which is a nice treat occasionally. They also control some gross (but mostly harmless) stuff that likes to live in the soil, like fly larvae. Butterworts are another option - I have some P. lusitanica that does very well in an undrained pot and goes to seed all the time.
Common, easy varieties of these plants (sundews, bladderworts, and butterworts) are very cheap retail, like $5 for a small plant, and because many of them reproduce readily, they're also available for free or trade on the forums. Most established growers have more seed/plantlets/seedlings than they can care for, and are happy to give them away to beginners. Larger, longer-lived varieties like Nepenthes and the other pitcher plants are the expensive items. But, still, many of the common Nepenthes like N. sanguinea, ventricosa or alata are readily propagated and shouldn't run you any more than $10 for a small plant suitable for your purposes. Someone might even be willing to give you a cutting to get started.
If you decide to buy a plant, buy it from a vendor that specializes in carnivorous plants - hardware store plants are often in poor health and are bound to die on you. If you do find some hardware store plants, don't take them home unless they're very healthy looking or you can talk the clerk into selling them to you for cheap... and don't feel discouraged if they die. I personally prefer to buy from CP vendors, because I don't have to worry about how the plant was cared for before I recieved it - I know it was watered correctly and not potted in bad soil. You'll often get a fairer price as well. That said, I would advise that you buy small plants as opposed to large specimens of the same species, because your terrarium will fill up fast and the plants will undoubtedly get much bigger. And besides, the more expensive your plant is, the more you'll kick yourself if it croaks. Wiser to buy a bunch of plants and find the ones that you can grow well.
If you want to grow a Nepenthes, keep in mind that most of them like well-drained soil that is airy and not waterlogged. Maintaining them in a planted terrarium can become difficult because terrariums tend to have a water table at the bottom of the substrate. If you want pitcher plants in your terrarium, you could get some small orchid baskets and pot them with sphagnum. Orchid baskets are perforated on the sides to allow air to flow into the soil, and with a little time you can get your ground cover to grow out of the sides of the basket, disguising it. Some people dig holes in their terrariums and sink pots into them for the same reason - a plastic dish sunk into the soil could keep your Nepenthes' roots a little on the dry side while the rest of the tank gets as wet as it needs. Another option is to build up one side of the terrarium with an airy mix for the Nepenthes, making sure that it's deep enough that the plant's roots won't be submerged in the water table.
These topics come up a lot, so if you dig around in the greenhouse forum here, I'm sure you can find more in-depth advice. If Nepenthes have caught your eye, you should start looking at the Nepethes thread, too, as Nepenthes can be picky about their conditions. Don't let that dissuade you, though; once your pitcher plant has settled into it's home, it should thrive as long as you give it adaquate water and constant conditions. You just need to learn their quirks first. And remember, there's no saying you have to get them all now - start with a few easy plants and get on your feet, because before you know it you'll have more than you can care for.
Have fun,
~Joe