If you think about it no one is going to manufacture an ER40 collet chuck to fit the small Myford nose thread because the mandrel bore is too small to make use of the collet capacity and few people are likely to buy them. Actually I think your best best is to buy an ER40 collet closer and the collets, and make the chuck body yourself. Use a big slug of nice steel, bore through and turn a register/nose thread at one end, turn round and mount on the nose to turn the collet taper and closer thread. Set the top slide to the collet taper by sliding a collet onto a nominal size ground steel rod and holding it in the 3jaw, then use a dti. It's relatively easy to make ER collet holders, I did an ER25 one a few years back.