I got one via the dealer I bought my 110 off, as the lock was faulty. Took less than 10mins to replace, as the old unit simply unbolted and was replaced with the new. Had to recycle the plates and nuts/bolts to fix it in place, and the lock barrel from the front doors also fits (took the chance to replace them all so all three match).
A quick go on ebay and google will trun up plenty, £20-30 seems the going rate.