What frequency range is acceptable for a speaker in an audio amplifier
Be careful with the power ratings of speakers. In the hifi community the power rating is not understood as a value where the speaker performs best. It's understood as a value which, if exceeded breaks the speaker or produces very bad sound results. When picking speakers, you usually multiply the power ratings by 3-4, just to be sure.
Before searching for speakers check your amplifiers total output power!
Regarding frequency:
10khz is not enough for a decent music quality. Low quality audio hardware ranges from 50HZ - 16khz.
Decent quality would be 18Hz to 20kHz.
Now, a single speaker usually does not perform well over such a wide range. That is why normal speakers are build of a tweeter (for high frequencies) and a woofer (for low frequencies). The woofer plays everything from 50-2khz, for example, the tweeter plays everything else. The signal is split with a high pass, for example.  

There is a solution for that problem as well:
Look for a broad band speaker. They are usually ranged from 50Hz - 16kHz. They are not meant to play to loud, though.
One more thing: Keep in mind, that even if a speaker is rated as 180Hz ~ 10kHz it won't perform the same on all these frequencies. Some frequencies will be louder, others are choked. Check the frequency response diagram for the speaker if available.

Another solution:
Try finding PC speakers, somewhere. Maybe even old ones, people dump them everywhere, for no obvious reason. You might know someone who can give you a pair.