I purchase limes from my local Mexican market. Those "mercados" that most of us pass by while driving. They are usually, by comparison, very cheap. The last bunch I bought was 10/$1 but have purchased the 20/$1 also. They also have key limes, those small ones, a 2 lb bag for $1, sometimes a little more. I have a small hand press for those, which I bought cheaply also at the store and you need about 15 key limes to make a half a cup of juice.
Limes are a staple food in the Mexican diet. They are abundant in Mexico and therefore cheap.
How to chose a lime for Juicing... the greener they are , the less juice. Juice the ones that have turned mostly yellow in color or have more yellow in the skin than green. When you buy them, purchase the yellower ones!They juice easier and a lot more. You can also try rolling the fruit on the countertop while applying firm pressure. This helps to release the juice and works for any citrus fruit. (if they are yellow you dont need to do this)
Are they organic... dont know. I wash them well though.