You can get practice papers from your examination board website but Im pretty sure you have to order them. They cost a little bit of money but not too much. Or you might be able to download them from the website. Have a look. There are links to the examination board websites in the sticky "Forum guidelines and useful links".
To download AQA past papers, go to http://www.aqa.org.uk/qual/index.html and select your qualification level and subject. Then select the 'Assessment Material' link from the top-right.
where on ocr site I cant see order form