Hi Stephane,
The way I get testimonials is by asking previous clients. However, what I've found is that the reason why many people do not write testimonials is not because they don't want to give you one, they just can't be bothered spending a couple of minutes making one up.
What I do, is that I write a batch of testimonials myself and ask clients to choose one from the list. It's more time saving for them and they will more willing to "give" you a testimonial.
Hope this helps.