Office Depot has 0.5 L collapsible bottles, with a "squirt" top, and carabiner for 1.09. Come in different colors and patterns, in bins near cash register
Yes the bottles are small for backpacking, weigh `0.5 oz ish, dont recall exactly, but the threads match a platypus , and the squirt top fits a platy exactly, and weighs 0.17 oz. It has a long collar that could be trimmed some too.
(If they only had 1L size, it would be awesome)