I use Jarrow Chlorella and Now Spirulina... I am not going to say they are the best, however, I do believe they are the best bang for the buck if you get them here;
If you find a better bang for the buck product, let us know.
You do not need a refractometer. Just be sure to buy the heaviest and deepest colored fruits and vegetables... weight/size ratio. They may not be the biggest or the prettiest fruit in the bin, but they will be better for you.