When you go to buy anything online you have to make sure you choose a reputable site and one that has a return policy, just in case the goods you buy are not right. You will need to look for signs that your chosen online store is safe and secure with the payment pages encrypting your payment details. Look for the URL starting https; the s on the end means it is secure. Another way to tell is the padlock symbol on the bottom right of the page. And don't forget to read all the terms and conditions first.
Once you know what part you need you can then go ahead and buy it. This need not be difficult if you have a computer and Internet connection. There are many spare parts stores online; all you need is the name and model number of the part to order it and it will usually arrive on the doorstep within a few days.