What you want to get you can get. Figure out if you want 2 channel, 4 channel or even 5/6 channel amps that come with crossovers, power and gains built for subs with your normal 4 speaks.

Ebay is a great option. Crutchfield or any other car audio review site will give you an idea on what to look for. Most anything will work but try and keep away from the really cheap ones... like most everything, you get what you pay for (but most people don't need the 2x25, stable to 0.25 ohm amps for $1000)