You might try contacting Dr. Diff to see if he would sell you just a push rod retainer. I got a new one with the adjustable push rod I bought from him.
Not sure where (other than a rebuild kit as TB mentioned) to get a dust boot.
Seriously.... you can cut the rod retainer ring from the correct sized piece of vacuum line. It is just unreinforced, soft rubber, and has no sealing function. The shape of the machined groove on the end of the rod is what makes a squared end o-ring work for retention.