Hello Alan,
Plates are available individually from our usual suppliers as well as a tailstock to suit a BSO head which is the model you have. You might also pick up a handbook that way if you haven't used the pdf link you were sent. The tailstock will be the same as one used for a 6 inch rotary table set on it's side. The centre height is a nominal 4 inches.
Alternatively look for a tailstock on ebay, they are very non precision things really in that you set the centreheight with cross bolts just to put the support centre on axis through the head. After that it needs no further setting.
Index plates also appear on ebay from time to time, useful to have as they may extend your set. They are easy enough to modify [with care] to fit your head too with a little lathework. You can accomodate plates up to 5 inch diameter and still get the dividing head to clear the table without having to use raising blocks.
That diameter will enable drilling for a ring of holes up to #69 and those can be reached with the indexing pin on your head by extending the slot to increase the arm radius.
The standard plate set you need are 3 in number to give
15,16,17,18,19,20 holes
21,23,27,29,31.33 holes
37,39,41,43,47,49 holes
The set will give you all divisions up to 50 and many beyond that are multiples of the numbers you have.
The spindle bore is actually a Brown and Sharp taper, but I have found 2MT fits well enough. The nose on yours will be threaded for Myford chucks.
Enjoy your new toy
Brian