Since I emailed you the other day, I tried ebay and also found a site
to look at. There are any number of scopes, both new and used on
ebay. I don’t like all the waiting and bidding, but it looks like
you might find what you want there. I would read the terms and not
buy without a return privilege. Also, it appeared that some of the
new scopes have illumination from below, but I don’t think it is
darkfield. It might work like darkfield if you set your gem in a
pair of tweezers above the light and put a disk (a blackened dime or
penny) below it so that light could only get into the gem from
oblique angles. You move the gem up and down until the light is
coming in at the best angle. A true gemological darkfield light
would probably direct the light into the gem a little better and
would also be stronger, which is only an issue for dark stones, but,
for them, you may need the stronger light to see anything.
I also ran across an item listed by Arc Micro Optics, and they have a
number of items for sale at their website (www.arcmicrooptics.com)
which you might look at. There are probably some other sites which
sell used and surplus lab equipment which might have what you seek.
Try a search or two.