Found this while looking for other things-Congressional document showing "all" sales of US Ordnance Dept. stores and equipment 1865-71. I found it interesting because the prevailing theory seems to be that Francis Bannerman & Sons bought up most of the Civil War surplus. Not true, they didn't seem to be much of a player, if at all, back then. Outfits like S., Hartley, and Graham could make offers to buy cannons and accouterments, for instance. I noticed one place in the long list where they bought four mountain howitzers with carriages, shells, implements, etc. The list contains a lot of cannons, some listed individually by caliber ("6-pounder") and some lumped into big lots (63,427 lbs. of old cannons.) When such were sold to foundries, we can be pretty sure they were turned into water pipe, pumps, whatever, and no longer survive. However, individually-listed cannons could still be around, privately-owned, somewhere. Maybe you can use the names of successful buyers in this list and find some nice cannons in some abandoned buildings. I found a beautiful pair of bronze M1841 6-pounder tubes in an old foundry about 10 years ago, or really, they found me.
http://books.google.com/books?id=q_ZYAAAAcAAJ&dq=Ames%20Hitchcock%20cannon&pg=PA460#v=onepage&q=Ames%20Hitchcock%20cannon&f=false
