Here is a list of high voltage transformers and where to get them.
Ignition coils- This should be obvious. See automobiles.
Oil burner ignitors- These are available very
cheaply (used) at a local HVAC dealer.
Neon sign transformers- Available from demolition
companies and neon sign shops. Ask for a used unit.
These are often repairable by removing the tar by freezing them and then
chipping the tar off the windings.
Flyback transformers- These can be had from TVs
and monitors that are left out for trash pickup, or
in you local TV repair shops dumpster, or maybe you can even get brave
and
ask them for one.
Microwave oven transformers- See flyback transformers
for places to obtain them.
Xenon transformers- These are very rare. They can be found in some laser
applications. They are current
limited like microwave oven, oil burner, and neon sign transformers, but
have lower output current typically.
Medical/dental x-ray transformer- These are very powerful transformers.
A typical voltage output of just
one of these is around 60-200kV at 50-120mA. These look like
they are very hard to obtain.
Power distribution transformers- These can be obtained from the power company.
They are very powerful.
There are several different types. Voltage outputs may range
from 7.5kV to 20kV at around 500mA to 1A typically. This transformer
is the "metal can" or "pole pig" which hangs on the
power
pole. They must be wired in reverse for step-up operation and
be externally ballasted. If they are not externally ballasted you will
probably cause a brown-out. A good current limiter is an arc welder
or a tub of water doped with baking soda in series with one
of the primary inputs.
Vacuum tube plate transformers- These are used in high power RF amplifiers.
Typically 1.5kV to 3kV
(of course many are below this). Look at a hamfest for them.
Of course you can always wind your own high voltage transformer if you have enough knowledge of transformer construction. Just break the core apart and wind a new secondary winding in reference to the primary turns-per-volt ratio. Magnetic shunts are also always a good idea.