Sorry, I can't see a manufacturers name on your piece. Most of the pieces similar to yours that I have had were marked Coro, but I think most of the major jewelry manufacturers were producing similar in that era (1940's).
Regarding a loupe, there are thousands online and some are not expensive, I have paid as little as 2-3 dollars and as much as $25, they all seem to do the same job. The one I use most often has 3 swing lenses in one case. 10x, 20x and 30x magnification. I keep one in my purse for when I am out shopping and have several others scattered throughout the house as I frequently misplace them. Some loupes are available with a loop at the top, so they could be worn with a chain around the neck, handy to wear one that way if working on jewelry for a session of identification.
