You should be able to purchase them through any quality shop who is an Aldan retailer.
I just did a quick Google search, Zigs popped up first, <a href="http://www.zigsstreetrods.com/suspens/rearsusp.htm#AldanCoilOvers" target="_blank">http://www.zigsstreetrods.com/suspens/rearsusp.htm#AldanCoilOvers</a>
The construction of the Aldan's are just better in my opinion. Tried them all. Aldan's are machined aluminum...not a stamped steel housing, they are fully rebuildable, etc. Many nice touches.
Here, they can tell you about themselves...haha
<a href="http://www.aldanshocks.com/aldanfeatures.html" target="_blank">http://www.aldanshocks.com/aldanfeatures.html</a>