You could try this site:
http://www.easywildflowers.com/quality/spigelia_marilandica_indianpink_.htm

Sunlight Gardens and Pine Ridge Gardens are both out of stock at the moment.

If you want a large quanity, i.e., 72 or multiples thereof,
here's a grower in FL that TC's them. I have seen their Spigelia on display at the SNA Trade Show and they are fairly large, healthy looking plants.
If you don't want that many, see if a friend or friends would share an order with you.
Best to call for availibility, as they don't always have everthing on their list in stock.
Rb

Here is a link that might be useful: AgriStarts3

If the sources listed above don't work out for you, try Gardens of the Blue Ridge (www.gardensoftheblueridge.com).
I bought some plants from them last summer (including 3 spigelia marilandicas) and was very pleased with them. The plants were healthy and of a good size and have grown well this spring. Unfortunately, just as the spigelias were about to bloom a few weeks ago, a deer lopped the tops off of them, so it looks like I'll have to cage them and wait until next year for flowers.

Thanks for all the replies - I'll be getting more shortly. I think this is my favorite plant in my garden, especially at this time of the year.

Bob