The output voltage depends on the transformer's primary:secondary ratio. They are unidirectional, so a transformer wound to supply 6V from a 120V source can be used to supply 120V from a 6V source.
The transformer design depends on your power requirements and voltages. The output has a centre tap to easily reverse the polarity through the windings (and thus create a basic AC input) without the use of an H-Bridge.
If you're looking to keep total costs down to under $15 per unit, something like this or this may be helpful.
To answer your final question, it may prove safer, more efficient, and less expensive to use a commercially available inverter and inexpensive car battery to power all 20-30 bulbs from a common source.
The linked inverter circuit is not terribly efficient, and the lower your input voltage, the more current drawn due to the conservation of energy. 3W at 6V means a 500mA draw from your cells, under ideal conditions. Increasing the input voltage decreases the current consumption, and extends the life of the batteries.
If you like doing this kind of thing, making one to test would be fun! That being said, if you're planning to host an event, there may be plenty of other things on your mind that day, and taking 'home made high voltage supply' off that list might help.