I was told that the AT coolers usually have smaller passages or at least smaller fittings that can't flow enough oil to use as an engine oil cooler.
I found an engine oil cooler in a van conversion (mini motor home) at the junk yard. I took the trans cooler too. Both are about the same size and style, but the trans cooler has much smaller fittings.
I would assume that an autotrans doesn't flow as much fluid as and engine does. It's just an assumption as I have no numbers to back it.
At the junkyards around here, turbo cars, diesels, vans and some pickups usually yield engine oil coolers.