There are a number of ways to get feedback and testimonials from customers and clients.
I have worked heavily in the Public Speaking industry for the past few years, i mention this because unlike some of the other industries I've worked with , here release so heavily on PR.
A few common ways are to use a rating section on your website to help for a wide cast net, or direct contact for more laser targeted approach. Typically if you pre frame the customer experience to end in a survey to acquire feedback you shouldn't have any issues.
If you do call me! I would love to help!