Variable Capacitor??

Yes there are such things, but only for relatively small values of capacitance.

A capacitor substitution box is probably what you are after - you can buy posh multi-decade ones (ebay often has them). Make sure it is rated for the voltages which you are planning to use it with.

Or you can simply get a handfull of standard values, adequately rated, capacitors and make your own simple substitution box.

Yes! there is such a thing as a variable capacitor, although as Palindr=E2=98=BBme said i've never seen a value above the pico farad range. Variable capacitors are much more common than you might think. They are the most common way to tune small radios. The next time you get your hands on a $5 radio or some price in that area open it up, i can almost guarentee there will be a small translucent box with a rod sticking out of it connected the tuneing knob. Turn the knob and look through the plastic box, you should see metal plates moving inside.

this url will take you to a site that tells you how to build your own (notice it's a radio site)

I haven't looked much at cheap radios lately, but a variable capacitor and its drive is likely to be one of the more expensive components.

I recently bought some FM radios at the 99¢ store. It uses a Philips chip of considerable complexity to provide the complicated functions using digital techniques. This includes a variable if strip! Performance is not bad. Its biggest drawback is that it steps through the band.

Bill