GARMIN stopped making these in 2001 so be aware that they will be <u>significantly</u> out-of-date. You would be much better off using that GPS as a back-up ( DSC input ? ) and buying a new one.
They used to occassionaly come up on eBay but I haven't seen one for ages. Also check boatsandoutboards here
There are two diffewrent types of G-chart. Normal ones and "micro" ones which can be identified by a preceding M on their serial ID. The two are incompatible of course so be careful which one you bid on.