Which product should I use after my hair transplant to strengthen my non transplanted hair?

Thats a very young age to have a hair transplant.  you should definitely be on finasteride as well as rogaine and laser devices.  I would recommend seeing an experienced physician who can setup a plan for you in terms of medical management of hair loss.

There are no products what will strengthen non-transplanted or transplanted hairs from a proven medical point of view other than Propecia and Rogaine.  PRP, lasers, vitamins, biotin, or $500 products you refer to are generally a waste of money.  

Saline spray is the only immediate post operative medication I recommend to help keep grafts moist and healthy and to reduce itching.  After I few weeks, I like to see my patients back on minoxidil and finasteride.  I use PRP with nearly every transplant, and other growth factors which I think are the key factors to a good result.  If you did not have growth factors and PRP they can still be given after 4-6 weeks post operatively.  

Hair transplants in a 21 year old reflects malpractice today as a 21 year old should have been given the course of finasteride (drug) which often reverses hair loss in people of your age. You should be on Propecia at the least and not worry about other applications. I would worry more about your future hair loss and the proper management of your hair loss. See a local doctor, not in Turkey, who can develop a MASTER plan for you as you may have made a terrible mistake.