You mean the repair string right? I get the stuff for cars off ebay then chop it up with scissors or a penknife to get a reasonable size.
In the majority of cases it works, but it’s not infallible ~ I’ve used it with maybe a 70% success rate, but you have to carry the needle tool with you if you’re going to use it.
Used superglue to repair some holes as well, though you have to make sure all pressure is released from the tyre… I forgot this once and got an eye full of superglue. 🙁
Probably obvious, but worth making sure you keep the hole at the top, as if all the sealant leaks out, you’re stuffed. 😆