You can order online. Check out http://www.thujagardens.com/GreenGiant-quart.html

abalk,
If you are going to purchase in that quantity, you will not want them shipped to you off the internet.
Visit a couple of your local nurseries near your home and tell them what you want. Ask them what kind of a price they can offer you on the trees and tell them you will pick them up.
They can order you the trees wholesale which is very cheap and you might be surprised at the price.
The larger the quantity they order, the better the deal they will get, hence, you will get.
Try it, you have nothing to loose.
Just be prepared to put a deposit down, that is an order a nursery won't neccesarily want to be "stuck" with if you change your mind.
Good Luck to you!

Art:
I would check out nurseries in the Lancaster County area. I have heard that many of the Philly area landscapers get their trees out there. A friend recommended Ken's Garden Center in Intercourse on Route 772 W. The phone number I have is 717.768.3922. If the nurseries are owned by Amish people, they may not be easy to find. Good luck!

There's a place along Rt 283 just west of Lancaster exits (and the Outlet Malls). It looks like a big wholesale growing operation and I'm sure they could be able to supply that many. Here in Perry County, there is Greenwood Nursery and they may have that many available. I would think Lancaster would be closer to you.

Wholesalers may be the way to go, but Musser Farms (mailorder)in Indiana PA has a great reputation and prices as well.