In the "old days" of real estate I used to send a paper request to clients requesting feedback after a transaction had closed. Fast forward to now, and I'm using SurveyMonkey.
SurveyMonkey is easy to use, both for the sender and the respondent. There is a free version allowing one to ask 10 questions.
We'll be switching up the questions a bit; I want to give links to Google Reviews, Yelp! and Zillow when I ask for testimonials. (But, I certainly won't change Question 2: What can we do to improve? I love the answer here--it made me laugh out loud!)
Next, I want to figure out the best way to get video testimonials. There's always something new to discover and learn, but for now, SurveyMonkey works just fine.
Have you tried it?