When learning a language, many people use bilingual dictionaries. These are very useful, but once you get passed the basics, it’s a good idea to invest in a monolingual dictionary in your target language. When you look up words in bilingual dictionaries, you rely on your native language to understand what they mean. In a monolingual dictionary you have to rely entirely on the language you’re learning. This helps you to pick a lot of new vocabulary.
The only problem with monolingual dictionaries is finding places to buy them. There are monolingual dictionaries online, for example I just found a good Spanish dictionary here, but finding printed ones is not quite so easy. Any suggestions where to look?