These ultra-slim, pre-mounted e-textile LEDs from Kitronik run off a 3V coin cell battery and are illuminated continuously when powered. They have connections to which the conductive thread can be connected. These are clearly marked with '+' and '-' to indicate which is positive and which is the negative connection. The PCBs are double sided making the conductive thread connection very reliable (see example picture below).
This product is designed and manufactured in the UK by Kitronik.