actually a good portion of the meat birds you get are fast growing cornish rocks. they have a slow growing variety as well. if you want them to live beyond 10 weeks then do like i do. give them asprin with their water. plenty of soluble vitamin (powder doesnt dissolve well and is loaded with salt i reccomend red cell - horse mineral) and restict thier diet. ive kept them for 8 months before i finally butchered them. im going to try to keep some indefinite. heres a hatchery here in texas that sells both variety
http://www.xtremegamebirds.com/Meat Birds.html
http://www.xtremegamebirds.com/Meat Birds.html