Cobraplant.com is great.

black jungle terrarium supply(blackjungle.com)

Theres alwasys californiacarnivores.com I just got some pygmy sundews from there and they really impressed me with their plants.

I've also bought from blackjungle.com (got a cephalotus from them last year)

If you can't find what you want at either of those you could try hirts.com they sell a couple pitcher plant types certain times of the year.

I've bought from all three of those and been happy with my plants.

What type of pitcher plant are you looking for?

Technically, if you want an inexpencive plant to test out, you can try lowes. They usually have some in store in little cubes. They'll take a little more care and need repotted from the cubes but they're a good way to cut your teeth on growing one.

What type of pitcher plant are you looking for? American Pitcher, Sun Pitcher or Asian Pitcher? That may help narrow down the best places to look too.

I would also recomend, if you're an avid library fan, see if your local library has a copy of "the Savage Garden" that'll help you get through alot of the questions on care and species.

Here is a link that might be useful: Tangled Web of Aracknight

Here's a couple more other nurseries that may work out for you.
Lee's Botanical Garden, lbg-cp.com and Cook's Carnivorous Plants, flytraps.com.
I had good experience with them and have responded my e-mails. I'm sure they can answer any questions you may have.

This website is interesting:
http://www.agristarts3.com/Plants.asp?ID=2

But I doubt you could possibly buy from them until somehow you had a tax exempt certificate and bought in bulk. But they sure have cheap prices.