I have naturally curly hair and I use Garnier Fructis Sleek and Shine. It’s cheap and it works really well. They sell it at places like Wal-Mart and Target. 

I also use HEAD Organic Smoothing Serum, once the hair is dry, to tame any left over frizz or curls.

It’s never perfectly straight without a straightner but it stays pretty straight when I use these both together.