Most houseplants are grown from cuttings or divisions of larger plants, so I've never seen any place that sells the seeds. I think that a packet of seeds would be just as expensive as the small pots of tropicals (great for terrariums) at places like HD and Lowes though, so why not shop around? I know that you can sometimes get spores of mosses and ferns, but they can be very tricky to germinate. HD sells 'table ferns' (various) for about a buck though. That's how I got mine. Seeds of CPs are readily available. I think Cook's sells some, and you could inquire further on the CP forum.

Contact me by clicking on my name. Each month I throw away or sell cuttings from my terraria . . . although shipping during this time of the year could be tricky. Let me know what you are interested in growing, but realize that you will need a terrarium into which the plant cuttings should be transferred. Otherwise, plants taken from a high humidity location are likely to turn a bit crispy and die when suddenly transplanted into drier environs.

I had the same questions, so I researched the internet and found a website that offers instructional videos on this topic. The videos helped me a lot, thought I would share the information.

http://homegarden.expertvillage.com/videos/terrariums-planting.htm

Like you, I operate on the strictest of budgets. I keep an eye on the distressed plants at local home stores. I've picked up viable plants for pocket change that just need the slightest TLC. Just today, I went to Lowe's and picked up a pitcher plant for 25 cents. They marked it down simply because the box/container was cracked. I would recommend advertising on craigslist and freecycle, too. With a pleasant request, you could probably find local generous souls willing to contribute cuttings, divisions, or even whole plants. Finally, though it's late in the season for this, do a careful survey of your neighborhood. A variety of herbs and groundcovers make suitable terr plants. I picked up some coleus from a planter outside a bank after they'd passed they're prime. The bank was happy to get rid of them, and a well-pruned coleus makes a suitable pseudo-bonsai.

Don't know if these ideas suit your needs, but I hope they can help somehow...

Sometimes the "99 cents" store has plants for...oh well, you know.

Rare exotic seeds has some nice carniverous plants and tropicals.

Here is a link that might be useful: Rare Exotic seeds