Rogers tii has one listed under "Weber Carbs & Parts." I'm sure Pierce Manifolds will also have one. I purchased a universal choke cable from the local auto parts store that worked perfectly fine for about $10.
Though I've read that some don't have troubles with cold stars and don't need the choke cable, I found it made quite a difference in ease of cold starts. My engine starts cold on the first or second attempt with the choke; without it took 6 to 8 tries.