Buy cheap ones, I found a little extra grease added to the joint helps, and make sure the spanner you use to hold the flats of the ballpin have slim enough jaws not to displace the boot when tightening the nuts, I use a 15mm pushbike cone spanner for the job.
I fitted
THESE over 5 years ago, and have just had to replace one after a sharp stone pierced the lower rubber boot.
The rubber boots were spot on, no deterioration such as perishing.
Alf