RO water is pure, meaning it lacks those nutrients...
CalMag is necessary for you if you have RO, otherwise your plants will suffer from calcium and magnesium deficiencies.

I get my nutrients/supplements at Discount Hydroponics, and they have great prices... here's a link directly to their CalMag Plus page, but feel free to check out the rest of their selection too!

Discount Hydroponics - Cal-Mag Plus