"Only plant certified disease free stock!"
Is it ok to take some from a friend?
You could ask him if he has/had disease problems.
Also check if he is giving you "parent plants" rather than newly rooted runners!
The parent plants might be quite old e.g more than 3 years old! If you get these then the chances of getting disease is higher plus they will be weakened, usually I threw away plants that were more than 3 years old!
I must admit when I exhibited Dahlias & Chrysants we were very careful as both can be virused, so getting them from a fellow grower was generally safer than buying them from a nursery because we new their history! So my guess is; The plants you are getting are 'thinnings" i.e. rooted runners that are crowding out each other!