Capacitors (aka Stiffening Capacitors) are quite simple devices and can be lethal if used incorrectly.
I suggest purchasing from Cardomain if you can't find anything locally - I love this place, good prices, good selection. An explanation from their site:
Quote:
Most vehicles' alternators and batteries were truly only designed to power the stock electronics. Add a stiffening capacitor between the battery and your amplifier to provide your amp with a lightning fast on-demand power reserve.
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The general rule of thumb when selecting a cap is 1 farad for every 1000 watts RMS. For example, a 750 watt RMS amplifier requires a 1-farad capacitor.
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The concept is simple: Capacitors are energy storage devices. They store power and release when your amp needs to draw it. When you're putting a heavy load on the amp and you have that killer bass hit that would dim your lights or kill the battery, your capacitor will fire the power into your amp so that no draw from the battery and/or alternator.
http://www.cardomain.com/shoptype/Stiffening+Capacitors
I went ballzout on my capacitor. I bought the Phoenix Gold Powergrid. It's a 10 farad capacitor, which is probably ten times what I really need for my amps. I'd rather have too much than too little power though - anything to save my alternator.
If you wanted to go with a setup like mine, you'll need to use eBay because Cardomain isn't carrying it anymore. Just search for "powergrid". Here's one:
http://cgi.ebay.com/PHOENIX-GOLD-T-P...QQcmdZViewItem
I think I paid $250 for mine on clearance.
I would suggest going with one of these guys if you can't find a good price on eBay:
http://www.cardomain.com/item/STISWCKH42
It's $279. That's about what I paid for the powergrid...
If you're on a budget, Hooked On Tronics has this respectable round unit (most are round) here for $68:
http://www.hookedontronics.com/show_...s.jsp?cid=6034
It will work as well, but ... I prefer having something that is square and mounts more easily than most caps do. That's just my personal preference. That company (hooked on tronics) and Monster cable are both respectable vendors, so you can buy with confidence.
I recommend using a Gel Cell battery like the entire Optima line. They can handle being run into the ground and back up again. I would also consider buying a beefier alternator, just be sure that you have a suitable voltage regulator for it so you don't overcharge your battery, which could lead to an explosion.
If I were you, I would first replace the battery with a good Gel Cell battery. See if the stalling permissions persist. If so, your alternator might be nuked. Either way, you'll have needed the new battery. Once you figure that part of it out, go out and buy a cap for your amp. Actually - I'd separate the two - buy the cap now, install it, then figure out what's up with the alternator and battery - they're two separate issues.