IMSLP / Petrucci Music Library
This is one of the greatest resources available to us as musicians. It has scores of hundreds of pieces of music that have come out of copyright. Beethovens Symphonies, Mozart Concertos, etc. All of these are available as full-score downloads and often also as downloads of individual instrument parts. All legal and all FREE. Check it out!
The Clarinet Institute of Los Angeles
This is another great site. It has sheet music for solo clarinet as well as trios, quartets, and quintets. All of this sheet music is also copyright free. It's FREE to download and enjoy.