How to Find out which theme they use?

Wpthemedetector — free tool to find all the details about the WordPress theme used by a site.

Free:

For free Themes check out:

WordPress.org Themes — free WordPress themes directory.

Topwpthems — free quality and professional WordPress themes.

Fabthems — fabulous WordPress themes available for free.

Fthemes — amazing free WordPress themes site.

Paid:

You can Find different Themes in: 

ThemeForest — various WordPress themes and templates.

WooThemes — premium themes, plugins & ecommerce for WordPress.

ElegantThemes — beautiful premium WordPress themes.

StudioPress — professionally designed premium WordPress themes.

Themefuse — premium WordPress themes and WordPress templates.

Thesis Themes — the ultimate design and template manager for WordPress.

InkHive — premium WordPress themes.

RichWP — responsive premium WordPress themes.

Magazine3 — premium WordPress magazine themes.

Themify — drag & drop WordPress themes.

TeslaThemes — modern WordPress themes with clean design.

MyThemeShop — premium WordPress themes and plugins.

Press75 — premium WordPress themes.