• Play the bass clarinet part. This puts you in the right section, but you have to transpose everything up a fifth. And while the bass clarinet often plays the tenor line, the contra-alto is a natural bass instrument.
• Play the baritone sax part. No transposing here, but it puts you in the same range as the bass clarinet: not optimal for your instrument. It also may make you part of the sax section instead of the clarinet section.
• Play the tuba part. This puts you in the right range for your instrument (true bass!), but you have to do the Clef-and-Accidentals shuffle, and you may end up being part of the brass section, not the clarinet section.
• Play the bassoon part. The bassoon often plays the baseline for the woodwinds, so this may occasionally work, but you still have to do the Clef-and-Accidentals shuffle, and the part may be used to cover various tenor and alto lines, which are not in the ideal range for your instrument
• Play the string bass part. This puts you in the right range for your instrument, and tends to put you in the right section most of the time (the string bass often being written to bolster the bass of the woodwind section). But again you have to do the Clef-and-Accidentals shuffle (with an octave drop), and you'll find that in many parts the articulations (slurs, staccatos, legatos, etc.) are missing.
• Play the Bb contrabass clarinet part. This puts you in the right section every time. But occasionally it puts you below the range for your instrument, and you have to transpose everything down a fourth.
• Write your own contra-alto clarinet part. If you're adept at orchestration, you can create your own part, one in the right range, in the right section, with all the right markings. This is time consuming, but yields the best result.

Now, so that you won't have to do the same, I'm posting here my 'masterpieces'... in easy-to-print PDF format. If my concert band happens to have played a piece that you are playing, you'll be able to just download and print out your part, and then it will sound just right! (That is, if you play the instrument well.)

There is no profit to be made here, no fees to pay, no real market stolen. These parts will be of little use to anyone who hasn't already obtained a full copy of the piece. But hopefully I will have done my little part to promoting the use and enjoyment of this wonderful, versatile, and often neglected member of the clarinet section, which I believe adds a beautiful depth to the woodwind section of a concert band.

1. The parts of the score that you will likely base your contra-alto part on are: the bass clarinet, the bassoon, the baritone sax, the tuba, and the string bass.
2. Remember that the “bass” clarinet often covers the tenor line of the music. Sometimes your contra-alto part will differ quite a bit from the bass clarinet.
3. Remember that the bari sax and tuba often play in passages meant for brass instruments. A contra-alto clarinet is more purely a woodwind.
4. String bass parts often make excellent sources for contra-alto parts, because they often supply the needed bass depth for the woodwind section. Just remember that many of the articulations (staccato, legato, slurs) may be missing in pizzicato passages. You may have to look at the bass clarinet, bassoon or tuba part to get the appropriate articulation for the passage. The contra-alto clarinet can do an excellent job of obtaining the sound of a pizzicato string bass walking line, whether soft or loud.
5. You can sometimes write the contra-alto part an octave lower than the bassline. In somber passages, contra clarinets are able to play quite softly and gracefully in a range where other instruments would sound muddy or harsh. They can also play loud, complex contrabass passages more crisply than tuba or contrabassoon (the length of the cylindrical tubing of a contra-alto clarinet is half that of any other wind instrument of the same range, so it responds quickly in the low register). Just remember that this effect is best when limited to a few, carefully selected passages; a whole piece of groveling in the basement is contra abuse.
6. Many contra-alto clarinets out there are old and in poor condition, and don't play well throughout the entire range of the instrument. The written notes B natural and C in the middle of the staff (concert D and Eb in the tenor range) may have a tendency to squeak or squawk when preceded by lower notes. Notes above written G on the top of the staff (concert Bb near middle C) may not respond if the octave key system on the instrument is not in excellent adjustment.
7. In the clarion range (between written B in the middle of the staff and written C above the staff), the contra-alto in inexperienced hands may sound quite brilliant, more like a tenor saxophone than a clarinet, especially when played forte or fortissimo.