I agree, that plant doesn't appear to have any issues. You will see some leaves look a bit ragged as they age, and as the plant gets bigger, those leaves will shrivel and die as they have used up their usefulness. However, if you begin to notice significant yellowing and burnt look off the tips off the leaves while the rest of the leaf is much darker green, those are a sign of nutrient burn from having too much nutrient in their soil, or having nutrients added before the new roots have had a chance to get set and ready for them.
There are different opinions on when feeding should begin. I allow mine to grow without food until the little round "seed leaves"(also called cotyledons) begin to yellow as they are the food supply for the seedling while its roots are developing and hardening. When those little leaves begin to yellow, that means their job is done and they are used up, and the plant is now ready to begin to be fed with low doses of nutrients.
But yeah Amazon is easy enough. If you can't find what you need, I have several grow stores online that I use, just shout. 
