Some really good clarinet choir pieces are:
1.Clarinet Poem - Cailliet (I think that`s how you spell it) or any other clarinet choir piece by him.
2. Introduction and Allegro - Gordon Jacob (awesome piece - not hard but fun) or any other Gordon Jacob piece for choir
3. Seasonal Sketches (not sure of composer)
4. Andante and Allegro - Frackenpohl
5. Rockabelly - Paul Harvey (Middle Eastern feel that pays homage to very famous clarinet works such as the Mozart and Neilsen Concertos - its quite amusing to hear fragments of these and works incredibly well when paired with belly dancers in a performance)
6. Any intermediate arrangement of either The Arrival of The Queen of Sheba or Overture to Julius Caesar - Handel. The latter is better. An Australian clarinetist and arranger, Mark Walton, has arranged them so they may be available.
Do a search on the web for these - they are available. Just not sure of the publishers.