A -1.5 dioptre lens is nothing very special... many opticians would be able to provide a blank which cut be cut to size, although I'm sure they'll want paying for the job. But surely an optician must scrap lots of lenses? I'd talk nicely to your local optician.
Being a minus lens, it is of course concave rather than convex, the sort of thing used to correct short sight. Reading glasses do the opposite, they correct for long site (or at least the inability to focus close) so are of no use.
I've just searched for 'concave lens' on eBay and got lots of hits for all sorts of lenses... something may well suit your needs.