You don't have to join guilds to buy at guild traders. It's only necessary for selling to do so.
Getting them from another player (either bought or as a gift) is unfortunately your only option at present. If you play on PC EU, I'll happily gift you some writ, if still needed.
Btw, I'm already wondering why the "don't gatekeep endeavors behind difficult dailies"-crowd isn't here yet, telling us how terrible this endeavors are. Well, they are probably still busy arguing against this month's login-rewards. An entitlelists life seemigly isn't easy these days...
On a sidenote: I also still visit Los Santos every now and then. Also a great game.