Where do I get...: TV sets

(Links in general lead to photos of the component.)

From color TV sets you can get

30kV cascades. They are used to produce the accelerating voltage for the CRT.
Flyback transformers 8kV (peak-to-peak) which feed the cascades. For a photo see 8kV 20kHz transformer (upper left corner of the device).
in older models: one or two high-power, high-voltage transistors (e.g BU208) which are also useful for the 8kV 20kHz transformer . They are usually mounted on a large heatsink close by the flyback and cascade. In newer models, SCR's are used, which may also be useful.
If you're lucky, you may find a mains isolation transformer in older models (I saw exactly one such model up to now :-). However, if you find a large mains transformer, make sure it's really an isolating transformer!!

In b/w TV sets or computer monitors you can find

instead of a cascade only one hv rectifier (18-20kV). This may be a solid-state rectifier rod or a vacuum tube , if you've got very old TV. The vacuum tube is good for generating X-rays.
a flyback transformer with 18-20kV output. The rectifier may be more or less integrated into the flyback (and perhaps cannot be separated).
high-power, high-voltage transistor(s) as in color TV's. SCR's are not used. In really old models, there may be a valve (PL504) instead of the transistor(s).

Of course, there's a lot of other useful electronic components in a TV set, multi-purpose diodes/transistors, voltage regulators, caps, resistors etc. The CRT may also be of use, see "to do" section. The flybacks usually take an input of about 700Vpp half-sine-wave pulses (switched from 150V DC). Always look for a schematic, which may be attached to the case, it's very useful if you have to find out which terminals of a flyback you want to use and how the input should look like etc.

Remember that the CRT has a sufficient capacity to deliver a nice little electroshock, even after days. Short the metal parts on the outside with the hv terminal (look for cable from cascade/flyback) to discharge it.