OK, I looked at the web page for that transformer.

"It shows second voltage 30V+30V. What does it mean?"

This means that there are two secondary windings, each putting out 30VAC rms when the primary windings, of which there are two, are connected to 115VAC rms.

In your case, 30VAC is way too much. WAY too much as in fry your helpless car ams big time.

If you connect the primary windings in series (and the secondary windings in parallel) observing the proper polarities, then the secondary voltage will be 15VAC rms. This is still too much. This would give about 20VDC to the car amp. Poof!

I think one of the other transformers that have secondary voltage ratings of 25v, will be more suitable. If their primary windings are connected in series, then the secondary voltage will be 12.5VAC rms. Allowing for two diode drops due to the bridge rectifier, the DC voltage will be in the neighborhood of 16VDC. This may be too high for your amp, maybe not. It depends on what your car amp is rated for in regards to maximum voltage.

Parts Exprss also had diode bridges, but the highest current rating (continuous) was 25 amps, but I would want at least twice this. Other supply houses, such as Mouser or DigiKey may have higher rated ones.

As far as electrolytic caps, you could use the same cap that is offered for use in car amp installations. If you have a car cap, use it, otherwise, just bring $$$. Mouser or DigiKey (or Newark Electronics, or Allied Electroncs, or ...) would also have these. 20V 100,000UF would be a good starting point as far as capacitor value goes.

Paul