Pond tubs come in many different sizes but the most common sizes are available at Home Depot or Lowes. The largest rigid pond liner sold by Lowes is the 275 gallon MacCourt and the smallest sizes are only several gallons. Smaller pond liners are used for accents. Large pond liners are often irregular shapes to more accurately mimic a natural pond. Home Depot offers a very inexpensive 35-gallon liner that is easy to install.
Pond tubs sometimes have shelves for aquatic plants. The MacCourt 175-Gallon HDPE Pond Liner has a central platform for the placement of a waterfall or treasured plant. Multiple rigid pond liners are often combined to form a series of pools and waterfalls. They can be combined with non-rigid pond liners if a very large main pond is desired. Small 35-gallon plastic pond liners are great accents for smaller home gardens and they are very inexpensive.
The more shelves and the odder the shape of the pond, the more detailed the prep work that needs to be done in order to install the liner. Plastic livestock water troughs are often used for small ponds. Sometimes used troughs can be purchased for a bargain to make an inexpensive pond however the variety of shapes available is limited.