here you go:

http://www.garmin.com/us/products/onthetrail/custommaps

I’ve used handhelds for many years and have become a fan of Garmin. The latest units like my GPSMap78 also allow you take any .jpg image and create a custom map. So on small northern WI lakes I fish that have a paper map but aren’t on a chip, I can turn that paper map into a gps on-screen map. Amazing. Don’t have any experience with Lowrance, but as far as I know they don’t have custom mapping ability.