Where to download Copter 3.5 for an original IRIS+ with a PixHawk 1. (1Mb) to support the Gripper

Copter 3.6 has only dropped support for gripper on boards that have a 1MB bug.
Does your board suffer from the 1MB hardware bug?

If you try to upload a full featured FW to a board with the 1MB hardware bug using Mission Planner, you will get an error. So try it out to see if you have the bug.

One other way to find out is: 1MB Flash space or not

And yes, you can enable gripper support on 1MB builds, but probably you will need to disable some other feature.