It became pretty hard to find good and cheap variable capacitors, so people tend to build a bank of switchable caps, in order to tune circuits and find the right cap.
Here's a suggestion on how to do that most effectively:
To be able to switch the bank to ANY capacitance between 1 picofarad and 1 farad, you need only 30 caps.
You need 10 for the picos, 10 for the micros and 10 for the millis (if you really need such big caps). I suggest to get nonpolar ones that can handle the voltages you play with. You need:
1
2
4
8
16
32.
64
128
256
512
in picpfarad, microfarad and millifarad, maybe also nanofarad. Then you need to learn how binary numbers work, which is easy to understand. With this collection of caps you can switch to any capacitance, eg let's say you need. 90 uF:
that is 64 + 16 + 8 +2.
On the first look it may seem complicated, but in fact proximation with a binary search can be very effective.
Unfort. the common values for caps don't follow this binary philosophy, but you may find caps that are pretty close to those values.
Regards.