The fuses themselves will likely be available for a while. The fuse boxes haven't been manufactured for many decades. If you have a failure with the fuse box, it will likely have to be replaced. The problem is that it then becomes an emergency repair job. This almost always costs more than a planned/scheduled repair. You will likely need to have the entire service upgraded (not just the panel). This will also involve your local power company.
If your service is that old, then the wiring is probably 60+ yrs old as well. It is beyond time for an inspection. It is also probably time for a re-wire of the home. The insulation on wiring that old is likely in a very degraded condition due to time and other factors. Just because the conductors in your wiring are still doing their job, does not mean that the insulation is still doing its job very well.
Codes and laws do vary around the country. In my area (WA state), I have dealt with cases where insurance companies will not even write insurance for homes with fuse boxes or Knob-and-Tube wiring (This may also be present in your home). An insurance company does not have the authority to 'force' you to update the home. But, they may have the right to deny renewal of your policy, forcing you to shop for new insurance.
Costs vary greatly from one area to another. Consult licensed electrical contractors in your area if you need prices for this work. Anyone attempting to quote that in this column is a fool (since you do not state where you are at).
Sorry 'Harry', but you are nuts if you think that it only takes an hour and a half for a panel change and 3 to 4 days to re-wire an entire house to current National Electrical Code requirements.