I could not find anything suitable for the early to mid 18th century, all seemed to date to the American Revolution. So listening to my own advice of "earlier is better than later", I purchased a pair of 17th century reading spectacles from Trevor Timms in England (email@example.com).
These spectacles are sturdy to say the least, & there are no arms to get broken. They are secured to the head by a cord, ribbon or leather tie, so there is no chance of them falling off my head, and the optometrist had no problems fitting my prescription lenses.
My spectacles case is a copy of an original made by my youngest son.
The alternative to reading spectacles is the reading glass. The one below I made using an old magnifying lens, copied from an original including the case.