Amazon[amazon.com] has their AmazonBasics 4" x 6" thermal printer labels for $1.89 per roll or $7.70 for a 4-pack. Shipping is free when you spend $25 or more on products shipped and sold by Amazon.
I bought these the last time they went on sale for $2.03. Considering these are the cheapest 4 x 6" label rolls I've ever seen, I wasn't expecting much from them. To my surprise the quality of the labels and the adhesive is actually quite good. I'll definitely be buying more in the future (assuming the price doesn't skyrocket). Just to note for those who may care, the individual label backing is not perforated.
Update: Amazon raised the price on the four pack after SDers started placing orders. On the upside, now that it's over $8 it's no longer an add-on item.