Assuming, it is not likely, you can buy an Aero Ektar at your local camera shop, your only option is Ebay or the like on Internet.
We can divide the sellers in three categories:

#1 Camera stores around the world (Most of the time these are very conscious not to damage their reputation)- Generally no problems
#2 Stores with not so perfect reputation. - Need attention, please ask your questions by E-mail
#3 Individual persons selling their equipment. - Same as above!

Buying from a picture ::

• Unfortunately, detailed photographs seem hard to make, very often you see just blurred images or just one without showing the glass.- Don't bid!
• A very crisp photograph showing clear glass, nice finish can be tricky! Is this the real Item you will get?? See Example
• Avoid sellers, who tell they don't know anything about the item, or just selling for their Aunt who has no Internet connection. :-)
• Photograph does not show serial number - Ask the seller about it! The serial on the shade does not tell the whole story. (It is removable thus it can be from another lens)
• Serial numbers on the front part and the back must match!!
• Many lenses were tampered with, trying to dismantle without knowledge the parts and to clean eventually. This is very tricky, once the screws are damaged, there is likely damage on the threads also.
• Look at the sellers reputation and see his feedbacks. If you see any arguments or disputes. Back-off.
• Cosmetic descriptions are vague and lack important details. If there are dents, traces of fungus, damaged threads, glass or whatever, it should be stated.
• Unfortunately, not every seller acts according E-bay policies yet.

General remarks ::
What can be expected??

First of all, these lenses are not new and don't look like your modern Schneiderodefujikons! Having said this, it is possible to get a decent one. Look around at the images on these pages and see how they can look like. Some tiny bubbles in the glass or a little dust won't harm the final image.
The aperture/diaphragm ring should run smooth with noticeable clickstops that is. The yellowish stain on the glass of the back element is normal and can be healed (See elsewhere on the Site)
Very, very important, we must stress this, the threads should be examined. If the threads are damaged it will be difficult or even impossible to get a proper retainer ring on it! Check this with the seller.

Prices ::
Originally priced at USD 800 in 1940 and after WW II dumped as surplus at USD 5 / USD 50 were steady for many years. Starting about 2 years ago prices are rising/escalating again. Prices on E-bay from around USD 325 are no exception. I saw recently go one for USD 475! Is the Aero Ektar worth the price? I can only say you won't find elsewhere a lens with a starting aperture of f 2.5 and a 15 blades diaphragm.

Just before you plunk down any money, see for yourself which questions need answers. Getting in touch with the seller might save you from severe headaches. Happy hunting :-)
See here for an example of a bad transaction.