You can buy any generation SHO on E-bay, I think the 89 is listed by itself (it represents the 86 to 89 series) and the others follow the traditional grouping.
The biggest issue is getting the metal mast out of the casting that has the motor and the accumulator area for the plastic cable. Use lots of penetrating oil in advance.
Note which way the teeth are facing when you do the removal. To remove, just draw the ariel out by hand as the mast motor is running.
When everything is out, compare your old toothed plastic piece to your new one. If they are the same length, you can install your new mast. If your old one is shorter, there is some of the old geared plastic remaining in the accumulator, and the unit must come out of the fender to be taken apart.
The new one goes in the same way the old one came out. Feed the toothed plastic down into the hole while the someone turns off the radio (remember which way the teeth face from when you removed it).