I want to start out by saying that I respect the opinions of those that have posted, but I also think that the purchase of a green anaconda is a decision that requires a serious amount of forethought. While I am not an expert, I do have several years of anaconda experience and own both a green and a yellow anaconda. Both are females. My green is over 11 feet and weighs somewhere in the neighborhood of 95 to 100 lbs (it is extremely difficult to get an accurate weight on her) and she is around 3.5 to 4 years old. I have her in a 96 in. Neodesha with a 4 ft heated garden pond and overhead basking lights. This is the minimum for this snake, and keep in mind that she will need a much bigger enclosure with a much bigger pond as a full grown adult. I am just now in the planning stage for a "reptile house" in my back yard so that she can have the 10X8 foot enclosure with 300 gallon pond I feel would be appropriate for an adult that will be 16 feet and weigh another 50 to 100 lbs more. The point here is that a female green anaconda is a very expensive snake to house, feed, and maintain. Much more so then snakes of comparible length such as retics, Afrocks, and burms which can, in most cases, get by with a 96, or so, in cage. Also, anacondas are messy with a capital M. I also maintain several retics, African rocks, scrubs, and work in the reptile department at a zoo with large crocodilians, a retic over 20 feet, etc. and in my experience nothing comes close to what a green anaconda produces as far as feces. It will test your gag reflexes, trust me! However, on a positive note my green is very docile, and has never tried to bite or strike at me. She is also a very spectacular animal and is my favorite snake. I love her to death, but I also know that she could be exceptionally dangerous if provoked, so safety needs to also be in the forefront of your mind if you decide on a green.
Yellow's are smaller and have most of the same positives as greens, but with much smaller accommodations,expenses, etc. Case in point my yellow and green are around the same age but my yellow eats one 2 lb rabbit every 2 weeks and is barely 6 feet long. My green pounds 6 to 9 lb rabbits every 3 to 4 weeks and would gladly eat more if I let her. Yellow's, as great as they are, are not greens and if you must have a green go with a male as it will be a much smaller snake. Also, go captive born only, as you will get a much more docile animal without as high a risk of parasites, disease, and maladaption to captivity.
Best of luck with your anaconda,
Shane