Teleportation Circles are very rarely created by a lone magic-user. The rigor of casting the same 5th level spell every day over the course of a year is a difficult feat for a lone caster. Especially considering the cost (50gp of rare inks per casting, or 18,250gp over the course of a year). That’s why they are more typically found where a large number of magic-users might congregate.
Colleges, Universities, and Libraries where magical research is conducted are the likeliest places to find a Teleportation Circle. Add to this a few temples that partner with those who can cast this type of spell. The sigil sequence for these may be more well-known. Anyone who can create a circle will know at least a couple. All of these would have some safeguards against unwanted intrusion.
High level government facilities may have them, but the sigil sequences are closely held secrets. And the locations of the circles themselves are heavily guarded for fear of unwanted intrusion. These are fewer than one would think. The more people that know, the more likely you will have an intrusion issue. These are very rarely spoken of outside of the highest levels of government.
Due to the cost, only the wealthiest can afford to have their own circle. And that’s where craft and merchant guilds come into play. While most guilds tend to be secretive about their trade practices, the ability to move materials and people over great distances is an enormous advantage.
Guilds will typically have casters of the appropriate level to use a Teleportation Circle. Commonly used between two cities where the intervening territory is difficult to traverse either physically or politically. It’s not, however, very common for a guild to have a circle in every city they operate in.
There aren’t that many high level magic users that would allow for the efficient use of such a network. Remember, 18k gp per circle that takes a year to create. You would only really have a few where it’s really necessary. At any rate, sigil sequences for guilds aren’t commonly known.
Note that guilds need to retain magic users for continued service as having a circle isn’t enough to teleport. A caster is still needed to cast a Teleportation spell of some kind to make it work. And this, of course, costs money.
Finally, we come to the loners. Here and there someone is wealthy or powerful enough to make their own permanent circle. The lone hermit wizard in her home. The wealthy pirate in his grotto lair. The ancient dragon who has learned the sequences of dozens of locations over centuries.