Yikes! Having repaired and restored a number of these--including most recently building an 8506A out of two complete piles of rubble--I don't have a good solution. I've glued in remnants to make the modules fit, I've used the bonus ones from the unused CAL MEMORY slot that often don't get wrecked and so on. Unfortunately they usually break their retaining tabs off when handled. I wish they'd stuck with the earlier version without the retention mechanisms. If I ever do another one and don't have any of these parts, I'll probably 3D print some simple ones that glue in and just guide the modules in properly.
But good luck!