It depends on what's in the files, but you might have a look at Mapwel. It should be able to read the shapefiles and load them onto your GPS in Garmin .img format. That does not necessarily mean they will look nice, but it's a start and the software is pretty easy to use:
http://www.mapwel.eu/You can download the trial version and see what you think. The trial version will not export the fully detailed map, but it should display it correctly on your computer screen. Also, have you looked at the Garmin format maps here?
http://gpsmapsearch.com/