Where can I buy normal diodes?
You are looking for a Schottky diode. 
A normal diode would also do the job, but has about 700mV drop between the anode (the "input") and the cathode (the "output"). So if you put 5V in you get 4.3V out, which may or may not be a problem. A good example of a normal diode is the 1N4007, which can handle 1A and 700V.

A Schottky diode has a reduced forward drop, which is about 50% of a normal diode, because it uses a metal-semiconductor barrier. If you're interested you can read up on it on wikipedia. 

There are many distributors of electronic components out there. Farnell and RS components have UK warehouses, Digikey and Mouser ship from the U.S. All of them are good choices to search for schottky diodes but unfortunately they all charge quite high shipping costs if you order less than about 50$ worth. 

So I would recommend going to eBay and typing in Schottky Diode. Any diode with more than 500mA will do. Standard Voltages range from 20V - 100V. Lower Voltage and higher current is usually better (less forward voltage drop at your current). I'd recommend something in the 2A-3A and 20V-40V range. Don't use SMD parts unless you have a board to solder them to. And unless you have a lot of time don't buy in China/Hong Kong/Malaysia/etc. - it takes ages to ship to Europe.