A few places on Amazon sell single tubes for darts. [1]
The arborist throwline:
This needs to be fairly light — the darts aren't very heavy, so I can't use very strong elastic on the slingshot. But it also can't be too thin, otherwise it will tend to tangle way too much.
The most commonly-used arborist throwlines are 1.7 - 2.5 mm to minimize tangling. However, typical arborist throw-bags run around 300 g, so that kind of weight in front allows them to use much thicker throwline to minimize tangling.
Another requirement — it must be lightly or brightly colored, so it stands out among the background of the tree branches and leaves. It's hard enough to place a throwline, there's no need to make it practically invisible!
Stores that sell equipment for stunt kites often sell line that's similar-ish to what arborists use for throwline.
I've tried 1.0 mm Kevlar (with a break strength of 150 lbs), but it's far too thin, it looks like it will tangle if you look at it wrong.
I've tried 1.7 mm UHMWPE, hollow braid (with a break strength of 650 lbs / 300 kg), and it does work acceptably, but it seems like it's about the heaviest line I would want to use.
TODO: Next time try something like 1.4 or 1.5 mm. It's possible that's my sweet spot for the 20.0 g darts.