Amazon has their AmazonBasics 4" x 6" thermal printer labels for $2.03 per roll. Shipping is free when you spend $25 or more on products shipped and sold by Amazon.
Unfortunately, with this being an add-on item and a limit of 10 label rolls, you’ll need a filler item to reach the add-on $25 minimum spend. That said, this is one of the lowest prices per roll I’ve ever seen on 4 x 6 thermal labels (without buying huge bulk quantities).