These inline indicators are called "sighters" so you can search under that word.
nymphiing sigthers - Google Search
You can wind it around a small dowel or pencil. Color one side of the dowel/pencil with a broad tip black Sharpie and you will get alternating stripes that will be more visible.
Most euro-nymphers now use inline high vis yellow or orange monofilament for sighters. They cast much better than Dacron backing and you can match the mono diameter to the leader diameter.
Some sighters have fluorescent dots to make them even more visible.
You can make them yourself using fluorescent UV glue on your own mono as below.
Or you can buy them ready made.
You can buy them here.
WFLIES.COM: Leaders & Tippets