do you mean try to find somewhere that sells them as the HTC G1 is avaible form many networks.
or do you mean you want to find out where the phone handset is located..
you CAN do this if BEFORE its lost you pay and enlist on one of the s phone finder sites (subscription based and a confirmation that its OK to do this must be received form the handset you want to track)
I believe you are asking if you go on your computer to show you where your G1 is.
If this is correct there is an application called LOOPT that allows you to view on your phone or on the computer where you (your phone w GPS turned on) as well as your friends that have their phone's GPS on.