First read the manual that came with your product, and especially the fault diagnosis pages.
Approach the manufacturer: If the part has failed earlier than expected, ask them why. Let them know if you are dissatisfied. Check the terms of any warranty to see if you can have remedial work done under a guarantee.

Ask if they can support you to repair it yourself (e.g. supply diagrams or a procedure) or whether they have a service team available to do the work. Describe the symptoms and ask what are the failure modes associated with them.

Try to source a service manual outlining faults and remedies. These are often available online. The web is also a useful resource for discussion forums including products and repair topics for tradespeople. Here are a few sites:

How to mend a range of devices: www.howtomendit.com
Automotive and domestic repair manuals (even guitars!): www.haynes.co.uk
Manufacturers product manuals :http://safemanuals.com/

Once you have identified the likely problem, the spare and are convinced you can do the repair, locate a spares supplier.