The spindle thread should be 1 1/2" x 8 tpi.
It is the same as on Boxford and South Bend 9" machines - so there are quite a few chucks in circulation second-hand, but not necessarily cheap as these machines are so popular in schools, prototype shops and with hobbyists.
Many retailers will sell you a suitable chuck and/or threaded backplate - such as RDG (
https://www.rdgtools.co.uk/acatalog/BOXF ... _NEW1.html), Rotagrip (don't know if they have a website, but do have an ebay shop), Chronos (
https://www.chronos.ltd.uk), ARC (
www.arceurotrade.co.uk). Ask if they do educational discounts.
Or you can always try your luck on ebay!
Most 'affordable' new chucks today are made in the far east - I don't have any personal experience of using them, so my only comment is that the jaws are often much thicker than on a 'quality' chuck such as a Pratt Burnerd which can make a difference when trying to hold very small components. For school use, this may not be an issue.
For opinions on different chucks, you could try posting an enquiry (and a wanted ad) on
www.homeworkshop.org.uk
Hope this helps,
Andy