Upper ball joint boots fail quickly

I replaced all four ball joints on my 2007 Ranger about a 18 months ago. I grease them at every oil change (5000 miles). A few months ago, I noticed that the left upper ball joint boot was broken. It seems impossible to find a replacement boot. So I bought another upper control arm.

I changed my oil today, and replaced the left upper control arm. When I greased the ball joints, I found that the right side upper ball joint boot was broken too. I got good quality ball joints from NAPA. Has anyone else had similar problems?