Thought it might be worth mentioning that this cactus is extremely slow growing.
Not very common in cultivation in Australia as far as i can tell.
Plus the spination of seedlings is very vicious,
and they don't look anything like the adults.
also carnegiea tend to be expensive, and one a few feet tall is likely to be rather pricey.
i have a small seedling specimen, its hardly grown at all in over a year but i guess thats pretty typical. that reminds me i haven't watered it since like March-ish.
Definitely an interesting cactus and one of the essentials of any collection and a good talking point. Interesting history; used for many purposes by native american indians, and of course the most promoted species, used as scenery in movies. It is THE 'cactus' to most people.
Good investment too.
If you have some patience and are after authenticity go ahead and acquire one from the forementioned source,
but if you are after that saguaro look then there are hundreds of other columnar cactus species which will fulfil that requirement, in much less time and usually for much less $$.