High-Tensile Spidroin is a pretty difficult resource to get hold of in Starfield. There's only so long you can put off trying to locate some too, as you'll need it for gear upgrades later on. It is possible to find it when you're out and about, and you can also buy it from certain vendors if you're lucky.
That said, there is one way you can get High-Tensile Spidroin fairly reliably, and you don't need to meet any specific requirements either, so long as you're careful. With that in mind, here's where to find High-Tensile Spidroin in Starfield so you can start upgrading your gear.
Starfield High-Tensile Spidroin location
The most reliable way I've found of getting your hands on High-Tensile Spidroin is to harvest it yourself. As noted by Reddit user Tempestwolf1, this rare resource actually comes from a plant called Frigid Palm, and it just so happens you can find plenty of these in the Linnaeus System.
The bad news is that this particular system is a level 45 area, but I had no issues going there at a much lower level. The planet you're looking for is Linnaeus II and after scanning it, you want to make sure you choose a landing spot in the Frozen Mountains region. Once you've landed, look for a tall plant that looks like cacti—trust me, you'll know it when you spot one.
Once you've located a Frigid Palm, scan it, then get close enough for the 'Harvest' prompt to pop up, and gather your spoils. I've only been able to gather one from each plant, but they seem to be fairly abundant here, so you shouldn't have any trouble gathering a good amount.
It also helps that I didn't see any wildlife that wanted to murder me while I was there but it might be different for you, so be on your guard.
Buy High-Tensile Spidroin from vendors
It's apparently also possible to buy High-Tensile Spidroin from certain vendors, though I've personally not had any luck. The first is the Jemison Mercantile in New Atlantis (Alpha Centauri), and the other is Denis Averin in Cydonia (Sol). Neither of these vendors had it in stock when I checked on them, but even if they do have it, their stock is generally limited so don't expect to haul away armfuls if you do manage to catch them on a good day.
It's also possible to find the resource lying around at abandoned outposts and the like. Much like the vendors though, this comes down to a certain amount of luck so isn't a reliable way of sourcing much of the rare material.