Customer testimonials are a valuable addition to a website, they help to bring out the qualities of your products and services, and your own personal qualitites if you are a sole trader.
Sometimes a positive comment comes in unexpectedly through your mailbox, others may come from a feedback form or email survey that you send out.
You can also ask for a recommendation from a Linkedin contact you have worked with, which will be displayed on your Linkedin profile and can then be copied to your website.
Remember though, that if you work confidentially with your clients, you may need their permission to display their name, or use only their initials. And keeping a paper copy as evidence may be advisable.
So don't delay, update your website whenever you receive a positive review.