Hi, There is a place that sells it, Mulberry Farms, just don't linger too long at the site b/c they sell it for hornworms that are used as feeders to reptiles.
The other option is to make your own provided you can get all the ingredients.
http://www.entm.purdue.edu/Entomology/outreach/recipe/manduca.diet.htm
http://insected.arizona.edu/manduca/rear_diet.html
also try Carolina Bio supply and other scientific supply sources, but I think you will find it to be very expensive.
OR...you can raid your neighbors tomato plants! LOL! petunia, potato, green pepper and other plants are in the Solanaceae family so you may be able to find a generous neighbor or buy your little sextas a plant of thier own! 
lele
Mulberry Farm