Where can I get existing designs to practice on?

If you want to analyze existing HTML/CSS Layouts you can download a Theme from themeforest.net or similar sites.

You can see then how the PSD files are made, how the HTML structure is build up and how the CSS keeps it all together

All true, Technoir, but it’s worth pointing out that many of the best sites have no PSDs associated with them. The designers build them in the browser. Also, most of ThemeForest’s work is commercial, which means you would have to pay for the themes to download and deconstruct them. When there are millions of very good sites out there to examine for free, I see no reason to buy a theme merely to take it apart and see how it functions.

Yeah. Especially when…well, you can do that to any website on the web. (View Source)

~TehYoyo

Hi!
hmmm best option is to select the sites you like more and then use tools like firebug to get help for CSS and HTML, or if you want PSDs there are lot of sites which allow you free PSDs just google it but I suggest

www.dotemplate.com (also gives you HTML/CSS version of designs)
www.freetemplatesonline.com

hope it helps