What type of transformer do I need for a 5Hz AC source?
To avoid saturation at 100 volts and 5 Hz, a 50 Hz transformer would need to be rated at 1000 volts or higher. To avoid saturation with reduced frequency, the ratio of voltage to frequency must not exceed the ratio for which the transformer was designed. If that ratio is exceeded, the transformer will draw a high current with no load on the secondary and get very hot very quickly.

An old microwave oven transformer (MOT) might work if you use the original output winding as your input winding. However the ratio will probably not be high enough to make the secondary voltage as low as you want. You may need to connect the output of the MOT to another transformer to reduce the voltage some more.

Another alternative would to connect the primary windings of more than one transformer in series. Three 400 volt transformers would work. If you use three identical transformers, the secondary windings could be connected in parallel. Otherwise, connect them in series.

The above alternatives are not going to be very efficient.