Looking for a Switch....

I'm no expert on this ( homefully the real experts will give a good answer), but a mechanical switch with 100A going through it is likely to arc pretty badly and probably won't last very long.

Can you use a relay or a solid state switch instead?

You didn't state your voltage requirement. If it is below 600V, a manually operated fused disconnect should work well, & they're not that expensive. You may be able to find a used one from an electrical contractor in your area (they often keep equpment they tear out to use for temporary services & such). I suggest a fused unit to protect your test rig, and to allow the flexibility to use smaller fuses if you decide to test something with a lower current at a later date. You could also use a molded case circuit breaker.

I designed a 100A circuit breaker using 4 high current MOSFETs wired in parallel. International Rectifier MOSFETs share current well in a parallel configuration, with only modest loss.

Use a contactor which can be operated by on / off push buttons these have capabillitys for many hundreds of amps

Fuse switches of the rating or available for low voltage that is 600V and below.But problem is not clearly stated.I do understand what is meant by"hold 100Amp but not continuous".If not what is the duaration? is It frequent operation? and finally when nothing is mentioned we presume it is alternating current.

Use a mercury switch. Cheap, reliable, easy to configure.

Mercury switches work very well for a great many cycles. However they must be used with inline fuses, as they will explode, when there is a short circuit! I will not go into details but I once built a machine with outlet mercury contacts, which after several million cycles, with above rated amps, had no problems.

After the costumer had the machine for a while, I got complaints about the contractors not working. I flew to Mexico and found that they were testing the circuits by shorting the two outlet connectors together. If this contact was longer than a few parts of second, the contractors exploded! The mercury would vaporize and over pressure the the contact housing and explode. They must have put a vapor cloud into the air. This was many - many years ago when the dangers of mercury was not as well known.

I have not built a mercury contact machine since. If I ever build another you can bet there will be fast blow fuses on each outlet!