For a bought-new option: I buy these burlap sacks
every year to rake leaves into. They sit for a year and I get something like a decent leaf mold out of them. Usually by then the bottom has rotted out and I toss the intact scraps into the compost pile. Last year for my new pond I used some of the scraps to line plant baskets. They were pretty well done by the end of summer, but a) they were still holding soil in and b) that's after 1-2 years of perfect-for-rotting environments.
For a recycled option: I'd suggest looking at thrift stores after Easter for used baskets. Wood/straw ones will last a few years, plastic longer if you're not anti-plastic. Of course if you're looking for many of a single size, normal stores should have decent Easter basket sales too.
Edit: Re-reading the original post for comprehension this time
it sounds like you might want something that would hold more water in, where my pond baskets want water to flow easily in both directions. Wool felt should work nicely! If you know someone with sheep, you should be able to get scraps for cheap/free. Otherwise I'd probably go to the thrift store, get their ugliest/cheapest all-wool sweaters, wash and hot-dry them, and piece together liners.