Following a link on ReduceFootprint, I just ordered some heirloom seeds from the aptly-named rareseeds.com. The price per packet was comparable to that for non-heirloom varieties in my local store (I've bought 3 packets there already) and shipping was only $3.
I hope this works! Last year we grew some heirloom tomatoes from seedlings we got from a roadside stand, and they were very interesting! Our apartment's patio doesn't have a huge lot of space but everyone enjoys watching the plants grow so I think it's a win-win-win!