You can either find information about spare parts on the website of the respective manufacturer or you can contact the customer service. Often there is also information on spare parts in the operating instructions.
Especially with more expensive appliances, it is worth buying spare parts instead of a new appliance. For lower-quality models, you should consider whether it is worth buying a new unit.
A replaceable set of blades makes sense. Onions can only be chopped optimally with sharp blades. If your blades are not replaceable, you can sharpen them with a knife sharpener.