This is currently what I found that is currently new on the market for SepEx controllers. I know of no good used SepEx controllers available.
TSX 3.0 DC Controller | Waterloo, ON | Navitas Vehicle Systems
DC Separately Excited | Discover the TSX 1.0 600A 36-48V controller designed for golf cars, utility vehicles, material handling, ground support and low speed vehicles. Bonus features include the Navitas Bluetooth App.