If you can't find something in the secondary market, before deciding to purchase a new MT1 Jacobs chuck, you might want to consider this option:
If you have an MT2 Jacobs chuck, check to see how the chuck itself is attached to the arbor. In many instances, the chuck attaches via a J33 taper, and in that case, it would be less expensive to buy a J33-MT-1 arbor so that you can use your existing Jacobs chuck on your newish lathe. Grizzley sells replacement arbors for a lot less than the price of a new Jacobs chuck.