Some notes that might help you in your search:
1. The higher the sensitivity (rated in dB) that a speaker has, the louder it will play given a certain power input. Short story - speakers with a sensitivity of 90dB will play louder with a certain amp than speakers with a sensitivity of 85dB.
2. The lower the impedance in ohms of a speaker, the more current an amplifier will need to supply to that speaker. Some amps are not built to handle the current draw of low impedance (e.g. 4 ohm) speakers, other amps won't have any problems with it at all.
3. The exact numbers are sometimes debated, but some say that it takes an increase in 10dB to have something sound twice as loud. What this means to you is that it takes 10 times as much power from an amp to make something twice as loud. Most of the time that you are listening to music, you are probably drawing less than 1W from your amp. It's the transient noises (like cymbal crashes and such) that pull large amounts of power for very short periods of time. The point is that a 100W/channel amp isn't going to be significantly louder than a 90W/channel amp just because of the power rating.
4. You can certainly still buy two-channel amplifiers. You can buy one-channel amplifiers, in fact (they are often called monoblocks).
5. You could also use a receiver (stereo receiver or A/V receiver) to power your speakers. Not a bad option, IMO, as you might already have one around that you aren't using, or you could buy one used (or even new for pretty cheap...at least in the USA).
