If you have a Boaters World, West Marine or Boat US around, check there. If not, try Cabelas or BassPro. If none of them have the program and cables, then getting on Garmins website is probably your best bet.
I use Garmin's Map Source program with my computer or laptop.
I have an older Garmin unit (GPS V) but it works fine.
I find that the coastal maps are not well enough defined for navigating (definitely not chartplotter material), so you have to make your own routes, and then save them.
I find it mostly helpful for keeping track of where you went and marking channel routes for o' dark thirty driving.
I would recommend a Garmin product (over using web freeware!)