Montana 600 reformatting Garmin no longer supports GPS

[Reddeer]

Since Garmin no longer supports Montana 600 GPS,Is there away to download inland lake maps to the Montana 600?
The Garmin north America coastal maps bluechart G2 HXUSO39R doesn't have inland lakes. so 2nd part of my question is are there any inland maps for lakes that can de download or reformatted to work on a Montana 600? Peter