Joaquin69,
You might try and contact this company
http://www.gudebrod.com/ , I have some of their thread (flat) that a friend gave me. It is very thin and extremely durable. I am not sure exactly where my friend picked up the "roll ends" he has, but he also has blue and red, mine is black. I had included 12' in my PSK before Adventure Medical Kits released the Ritter PSK. If you want just a spool of the same thread found in the Ritter PSK, I would contact Adventure Medical Kits directly.
When you go to a sewing store and ask for the thread, as you have said, they won't always know what you are looking for, try asking for a thread used to sew canvas, or wool. Also, you may want to try marine outlet stores.
Take care,
Stargazer
Know your abilities. Know your limits. Respect them.