Simply type the word "Praktika" in the search engine (www.google.de, www.yahoo.de). Put some specifications to the search options and you will be able to get the information useful for you.
Other search options are:

• Information boards (Schwarze Bretter) of the universities,
• free magazines (Uni-Magazin, Unicum,...),
• specialised newspapers and economic magazines (Wirtschaftswoche, FAZ, Junge Karriere, Stern...),
• homepages of the companies...