This is called elastic thread. It came in white or red. It wasn't expensive so I got 10 yards. We'll see how it holds up.
The loose elastics I got from Fabricland and they were sold by the metre. Two of them are 1/4 inch and one bundle was 1/8th inch. The package elastic I bought at Dollarama.
The elastics in the middle I purchased at Fabricland too.
I decided to make four masks at a time so I didn't get to hate making them. Perhaps later this week I'll make four more.