Use old carpet. Watch for staples and tacks along the edges. Plush side up. Placing a bed of sand under the under-layment is optional.
If you are on stony /rocky ground, then a bed of sand is advised for extra protection.
If it is clay or regular soil the carpet is enough, though you can also use broken down cardboard cartons and layers of news papers or old tarpaulins etc, to boost the padding.
They all do the job.
"Horton"

Old member here. IMHO geotextile underlayment is a superior alternative for almost every installation. Carpet is a low cost alternative with some associated problems. 'Overpriced' is a matter of perspective; good liner certainly costs much more.

I just used weed fabric for my underlayment but my soil is very sandy and completely free of rocks.

Here there's always massive amts of old carpet lying out on the curb on trash day - esp. spring/summer/fall when people are renovating. You can "shop around" for stuff that isn't too gross or disgusting. Also posting wanted ads in Freecycle and/or Craigslist can get results.

We have clay so water can collect under the lining. Carpet began to rot right away and the smell was horrible. Ditto for cardboard and newspaper. Finally just made sure there were no rocks and smoothed the clay out. Works with no problems so far after 5(?)years. Sandy

I hit some very rocky soil while digging my pond. I also planned on planting some shrubs near the pond and was concerned with roots growing into the sides. I decided to use pond underlayment, which is similar to weed guard fabric. I also used two layers of professional (heavy duty) weed guard on top the underlayment and several inches of gravel. I think sand would have worked its way through the rocky soil and eventually disappear.

Although carpet is cheaper, IÂm happy with the investment I made. A tear in the pond liner could result in costly repairs, damaged pumps, and the loss of fish and plants.

I used a filter facric called typar that I got from a concrete company. It is a soft spun typar (NOT the woven type) that is quite strong and was about a dollar per foot but 12.5 feet wide. It is very similar to felt in thickness. I have a few rocks in the soil but nothing jagged. I put a double layer on the bottom and on most of the sides. This was cost effective and also very easy to work with as I think carpet would be hard to shape. If you go with the carpet check for tacks or staples and make sure the stiff backing can't curl toward the liner and cause a problem itself.

I used nylon carpet which will not rot for decades and also under layment. I have clay and jagged rocks so the carpet was a layer of extra protection.

old carpet for me. The weight of the water works out any kinks in the carpet in a hurry.