In my opinion Rusk or Phyto make the best relaxers you an buy without the help of a cosmetologist.
You can buy either Rusk or Phyto on Amazon.com.
Rusk is usually in the $20-$25 range and Phyto is in the $60-$70 range.
Rusk offers three different formulas ranging from mild to strong while Phyto offers two formulations.
I've known people using both products for years with awesome results.
If I were buying a relaxer for my hair I would buy the Phyto #2 which is their strongest formula. I like Rusk, but feel Phyto is just better for what I would like to accomplish with my naturally wavy hair.
I really can't recommend any other relaxers since I think some of the drugstore relaxers are way too damaging.
A drugstore relaxer might cost only $8-10, but if they make your hair break off or fall out, is it worth saving a few bucks?