The invisible Thread used on the Spools can be purchased at your local Sewing Center, you can pick up a spool for $5-$6 or you can purchase it from several sources online like Amazon. We highly recommend buying some invisible thread BEFORE you need it.
Amazon Link for Invisible Thread:
https://www.amazon.com/SINGER-260-Invisible-Thread-135-Yard/dp/B000IGEBSS/ref=sr_1_5?dchild=1&keywords=invisible+thread&qid=1606063670&sr=8-5