There’s another way. See alibaba... and contact a lens manufacturer in China for a quote to make a few.
Does it have to be glass ? Cast plastic ? How good does the surface have to be ? Scratches ?
https://m.alibaba.com/product/602915...732c2c6bHbRpAV
For example at $US30 apiece and minimum qty of 5 off, it would cost $150. Though the quality will be like that for cheap magnifying glasses.
A single lens to high optical precision will cost about $1000 though for 1 piece.
Part of the reason for the minimum order qty is that the Chinese factories are using fully automated grinding and polishing machines that work in batches and the min qty will be 1 batch.