In many areas (ours, for example) the stuff is pretty common, so I expect people who sell it just harvest it from a well-infested tree. There's one tree along the street near where I used to work that every year is just full of it, so at Alban Arthan I grab a friend with a ladder (hi, owl!
) and we go out and gather a bushel or so of the female plants (the ones with the berries) to supply our grove for our ceremony, the local Unitarian church for their Solstice ceremony, and all our friends who decorate their houses.
Here's some interesting information about mistletoe in North America: http://www.usgs.gov/newsroom/special/mistletoe/
. If you google mistletoe plants for sale
, you'll find some, and if you get plants with berries, you'll have the seeds. Be sure to order the species that grows in your area of the country if you want to try to propagate it.
Good luck with your project! I hope you have lots of success!