As this is only my first one, I don’t really have a “source” for the depth maps yet.
This map in particular came from a fishing book that I had already owned personally. Michigan has several fishing books for different sections of the state that include depth contours for the lakes. If someone orders a map for a lake not in this one book that I have, I’ll likely just purchase another book for that area and hope I keep getting orders.
I scanned the lake image and then traced the contours and roads into paths in Inkscape.