At first you go to a website name wordpress.org. In this site u find a lot of free wordpress themes.
No need expend money to use those themes. They are completely free. you can use these themes you ur website without expending money.
Premium Quality themes also get here for free :
After going to wordpress site -> click on Themes Tab > You See a lot of themes.
Now type your required/favorite themes name on search box and then press enter .
you can see a lot of themes here.
Click on Download Button.