First of all, if the GPS18 and the associated GPS software you are using meets your needs and is still currently functioning, don't mess with it. It will still work fine with newer maps. All you need are newer maps for the areas of interest.
This site will get you free maps for pretty much anywhere in the world. http://garmin.openstreetmap.nl/
Zoom in on the desired area to be able to better select the sections you want. It's worth spending a bit of time to understand the site and how to get the type of maps you want (e.g. to use in Mapsource/Basecamp on the PC for planning or to use directly on the GPS unit for navigation) but it's priceless when you know how to get the most from it.
I used it to get maps for tracking our Caribbean cruise a couple of years ago from New Jersey to the eastern Caribbean. On the ship I used Mapsource and nRoute on a netbook with the Garmin GPS 10x Bluetooth GPS. I also use the site to occasionally refresh my maps of western Canada. I have a Garmin iQue 3600A that can no longer use the newer format of City Navigator maps but works just fine with maps built from the OpenStreetMaps files.