If you look at Barnes and Noble, amazon.com, or Borders online sites and search for colloquial dictionaries you will find books like:

The Oxford Dictionary of Idioms

NTC's Dictionary of American Slang and Colloquial Expressions

Heavens to Betsy! and Other Curious Sayings

Slangman guide to Street Speak 1

Companies that publish ESL textbooks usually have dictionaries in their catalogs that would meet your needs. Remember that British, American, and Australian slang are all different, so you probably should decide which is more useful for your students.