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Depending on the voltage and current that is necessary for your circuit, you may want to consider using a voltage regulator, which will, you guessed it, regulate the voltage in to your circuit. The voltage regulators are designed to filter out voltage spikes which may be caused by the alternator. For instance, the LM7805 IC will accept a max input voltage of 35VDC and output at 5VDC at 1 amp. Similarly, a 78L12 (or Lm7812) provides an output of 12VDC. There are also voltage regulators designed for the VDC negative side, if you wish to filter the negative input as well. Also the LM317 is designed to be an adjustable voltage regulator, which you can set the output voltage that your circuit requires, and the circuitry to setup this adjustment capability is very minimal (1 resistor, a 5K trimmer and a couple of capacitors.) Please be aware that the voltage regulators will emit heat for the amount of voltage that you are filtering from the supply and the amount of current you are drawing from the regulator circuit. Thus, proper heat sinking is required in order to keep your circuit from overheating, though, the heat sinks required are generally very small, unless you are running a high VDC in (28 volts or so) and regulating that to a low VDC out (1 volt or so).
I'm not sure how to filter the supply only using capacitors. I have heard that voltage spikes from automotive alternators can reach 60Volts or so. Although, I am assuming that aircraft alternators are regulated more so than an automotive alternator would be. With that said, I'm not sure what type or size of capacitor will do here.
Regarding the photocells, have you tried Digikey.com?
Finally, here is a link to a site that uses the LM317 as a power supply IC for ground operations, but the methods can be applied to your situation.
http://electronics-lab.com/projects/power/010/index.html
Here is the link for the datasheet for the LM317. Similarly, you can find the appropriate link for the datasheets for the other voltage regulators on this site.
**broken link removed**
Good luck!