FYI:
99% of unwound fluoro low-G strings are going to be either 0.0358" or 0.0413" in diameter.
99% of WOUND low-G strings are going to be either 0.028" or 0.030" in diameter.
The wound strings are a higher density due to the over-wrap of silver-plated copper wire, thus thinner in diameter and slightly better intonation and tone but with about 3 lbs more string tension than the fluoro strings mentioned above.
ANY Classical guitar "D' string will work, and is the same in fact as most uke 'single' low-G strings of the same material.
If you do a search for 'thomastik' you will find LOTS of discussion of using this brand of CHOME flatwound strings, which many find to be an improvement in near-zero string squeak and finger noise over the Fremont Soloist wound gold-colored low-G string, which only sold as a single. The Thomastik strings are ALSO available as a single.
D'Addario single "D" classical string are usually ~$1.50 online, the Fremont $5, and the Thomastik $6.
Many many times, I have used a few careful strokes of a hacksaw blade to widen the nut slots, but be VERY careful since most hacksaw blades will remove LOTS of material in a single stroke, i.e., they 'cut' much faster than a file does.
I then use the welding tip cleaners, also with ONLY a few gentle strokes to make sure that the bottom of the nut slot is round.
Also, make sure that you are cutting at an angle down towards the tuners such that the breakpoint of the string over the nut is at the face of the nut. If you cut a slot that is parallel to the fretboard, you are going to have bad problems with buzzing and intonation.
+1 for the severe warning to NEVER use a triangle file for nut slots, unless you actually enjoy self-torture from intermittent tuning variations in response to the tuner-rotation vs. the tension that ends at the V-shaped nut slot, that is not immediately or EVER transfered to the vibrating length of the string because it is 'bound up' in the valley of the V-shape, while also constantly replacing strings from this same excessive tension in just a few short inches.
Amazon, ebay, Walmart and CB Gitty all sell a set of gaged welding-tip cleaners for $6 or less and they come in a holder that lets you flip out the size needed while keeping the other sizes inside, much like a swiss-army knife, and they are easy to use, but they do have quite a bit of flex, so you need to make sure how your hand moves when you gently stroke the nut slot so as to minimize the flex, as you are merely polishing the slot round and not sawing at it, so lots of force is not necessary.