The phrase "cheapest source" refers to the most affordable or least expensive option available. It is used to describe where one can obtain something at the lowest cost. Full definition
If you're on a tight budget, look for cheap sources of protein such as canned tuna, ground beef, whole eggs, frozen chicken breast, and cottage cheese. (fitnessandpower.com)
I will need a much cheaper source for almond flour to even try this recipe. (elanaspantry.com)
Any brand should be fine and you can find cheap sources. (nutritionfacts.org)