Ms.Starry, not sure where you are but if you are near a Cort Furniture location they sell name brand used furniture (including Aeron chairs). Good luck finding one that works!
I actually dislike the Aeron; I'm short and even the smaller sized one just never felt comfortable. My current office chair though is awesome and by far the best chair I've ever had with excellent lumbar support. If it didn't cost like $600 I'd consider buying one for home: www.knoll.com/products/product.jsp?prod_id=872.
The best office chair ever is the herman miller aeron chair. But I think it is like $1k.
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I did a ton of research before I bought my chair, and learned that the Herman Miller Mirra was just as good as the Aeron, and is $200-$300 less. I think I paid in the ballpark of $700 for my Mirra chair in 2011. It is amazing and was worth every penny.
But if you are going to go the Herman Miller route, you need to buy the fully loaded version and not the basic (which is priced a few hundred less). Do not give in to the temptation to buy a basic version, most reviews I read considered them a rip-off because they don't have all the adjustable features that make an ergonomic chair an ergonomic chair.
People who think Herman Millers are uncomfortable probably sat in a basic version, not a fully loaded one.
i am 5'-2". the most comfy task chairs i've sat in: vitra id (best looking out of these) knoll life (next best looking) haworth zody (but it's kinda fug, but way more reasonably priced than the rest of these 4) steelcase leap (also fug)
as far as herman miller goes, aeron is WAY more comfy than the sayl or mirra for me. some find the caper very comfy, and it's way cheaper than all of the above. all are fug. the bottom of the line aeron is SO uncomfy for me, but the a size (smallest) with all the bells and whistles is actually pretty comfy once i adjust everything correctly to my body.
i don't know where to get just one, though. lol. if you're in the LA area, there is a dealer that will just get one of these for non-business customers. you can probably get most online now. room & board and design within reach will carry some of these in stock if you want new.