I use Nokia Suite, altough when I back up it is usually all rather than contacts alone, and never neede to reload yet.
Although I can not go to download sites from work (barred from them) I have used 3rd party ones in the past. The ones I used would allow you to export the data to text or spreadsheet format files.
Suprised you can not keep the copy of contacts without a permanent connection though. Once they are backed up in a file I can't envisage where they would go.
Failing everything else I'd suspect stores such as Maplin would hold something you can plug in in needed, not that I've checked.