You can usually buy decorticated cardamom seeds at Indian grocers -- probably your cheapest source -- and I'm certain the Spice House would have it.
It'll remain fresher than the powder. Note the word "decorticated." You can try to scrape the seeds out of the pods if you're patient, but they'll be in a sticky resin. If it's only a little you need, and you're going to mortar and pestle it, you're probably fine, but if you need any quantity, try your neighborhood Indian shop.
What is patriotism, but the love of good things we ate in our childhood?
-- Lin Yutang