Buying locally used is a great option. Just make sure to be safe (duh) and individually demo the speakers before you buy. Buying affordable speakers used online is tough due to shipping costs; they can easily run $50-$100 or higher which kind of kills the value.
Our own sidebar guides are a great resource for bookshelf speakers/ and tower speakers.
This roundup of bookshelf speakers at TheWirecutter is a good place to get started. There are some notable omissions, but it's fairly comprehensive and (most importantly) it offers some background about what to look for.
Speakers (Compact)