I purchased a Clear Glass Crystal Ball 80mm (3.1inch), Photography/Display Crystal Ball from Amazon for around £10 a few years ago (sold by JJPRIME - but lots of others around the £10-£20 mark) and for the small amount of time I use it have been more than pleased with the results: e.g. https://www.blipfoto.com/entry/2422104058752928662.
The only thing I have noticed it that it does scratch fairly easily, but a "genuine" one might suffer the same, and you don't really see them. I keep thinking of looking out for some special polish as I suspect they would polish out.