If you are unlucky with part numbers, you could make yourself an encoder.
Just use a bunch of diodes to make one, and connect some anodes together.
As for finding an encoding chip, the best you will get is either a 8-3 encoder or a 16-4 decoder. The 8-3 encoder converts a number from 1 to 8 into a 3-bit binary number. The 16-4 encoder converts a number from 1 to 16 into a 4-bit binary number.
Try Nigel's suggestion first and make sure that the top 4 values are pulled down. That way you get an encoder from 1 to 12 instead of 1 to 16.