You can get such domains at a lot of places, includin eBay and various domain name forums like dnforum and namepros. I think often yo can get them cheaper before expiring, cause people want to sell them even for cheap instead of letting them expire.
Many place you can find. Godaddy auction, forum, ebay, many other. My way is find good name at deleteddomains or will be expired. Then buy at godaddy auction or other like snapname. godaddy auction is cheaper than others.
I used godaddy auction and could grab some of the nice aged domains at very competitive cost. I think you should go check it. Other alternatives are sedo but, you may find it very costly. sedo.com's no reserve auction is currently ON. you can check it too.
Before choosing one keep in mind the one which suits better yours and which is the most reliable! many such as (STOCKLii ) and those citd before but all are not the same!
One of my favorite places to find DECENT QUALITY aged domains is Godaddy Auctions, you can find some really good aged gems there. I have some from the 1990s thanks to them. eBay and popular domain name forums can be a viable option also.
you can go to sedo , yourmaindomain.com,getdotted.com and flippa to search for the old domain seller.