V.B.
I've never heard of buying insurance for glasses EXCEPT when you purchase them initially. Often, the vendor will offer insurance that you buy AT THE TIME OF PURCHASE.
If they don't mention it to you at the time, ask about it. I've always had it offered to me on any new glasses purchase, both for myself and my daughter.
For example: http://www.searsoptical.com/en/so-us/eyewear-protection-plan