Large Sarracenia for sale aren't very common, and they're fairly expensive (to purchase and to ship).
+1 on @Supercazzola's recommendation. Mike Wang has a fantastic
collection of Sarracenia. However, even Mike is out of stock on many purpurea's, and the only large one he has is a "marl bog form" from Ontario, Canada (that would be ssp. purpurea, not ssp. venosa). You could always shoot an e-mail to Mike asking if he has a large venosa that he's willing to sell.
Calen Hall also has some nice Sarracenia
for sale. Again, no large ones. I have his S. purpurea ssp. venosa Tatnall County, Georgia and S. purpurea "Veinless" plants, and I'm very pleased with them. It'll take a couple more years to get them up to large size, though.