The A-Frame Level is a common permaculture tool for a permaculture designer. This easy to make tool is intended for on-contour projects. Earthworks projects such as swales, ponds, hugelkultur mounds, and food forest planting, plus more are made on-contour. An A-Frame Level can be made out of leftover lumber or lumber can be bought. Straight pieces of debris from trees or shrubs on your property can be used to make the A for the A-Frame Level, which is the least costly option. The level needs a plumb-bob or an object tied to string at the top of the A to center itself into the center of the A to identify the contour line as you move across your property with a flag or another object as a market. I've seen A-Frame levels with bubble level at the center line of the A-frame to be extra certain of the calibration.
Watch this video as an example on how to make your A-Frame Level out of tree or shrub materials from Oregon State University.
Formal Plumb bob for A-Frame. A Toolbox bubble level can be put onto the A-Frame Level to confirm the calibration of the level at the mark made on the center of the A. There can be a small bubble level permanently attached to the A-Frame Level too. (bubble level pack)
The Johnson Level - Hand Held Sight Bubble Level can be purchased online. It's small, but needs at least two people to properly mark the contour line on your land. An A-Frame Level is easier to use, as it only needs one person to mark contour lines. The Johnson level is much easier to transport, however.