Great find mark!.
One day i was at a local photo store looking for a lensboard for a crown graphic. The sales rep was a bit clueless and came back from the storeroom with a 4x5 graflock back. I told him that wasnt what I was looking for, but asked how much it was anyway. At $20, I had to get it.
Another time I was at a surplus electronic store. I noticed an odd item, a panoramic security camera. After opening it up, I found out that it was actually made for 31mm x 120mm exposures on 35mm unperforated long rolls. The camera was unusable, but it had an Ilex 65mm wide angle lens similar to a super angulon. Took me years to find an ilex #1 shutter for it. But at $30 for the whole unit, I was extremely happy.
Now i've heard people find great deals at thrift stores, but I've never found anything good in the thrift stores around me. And i go like once a week.
Ive never thought to look in the drainage ditches though. I will have to add that to my usual rounds. Who knows what I will find!