The different sorts of mage robes that I found all have Fortify Destruction along with Magica Regenerate. The enchantment that appears in my list is
Fortify Destruction & Magica Regeneration.

So the question is - how do I just get Fortify Destruction? And If you can't does that mean that enchantment is only of any use once you reach 100 in Enchantment and can dual enchant?

I found a Ring of Minor Destruction that has the fortify enchantment alone.

Putting the "Fortify Destruction & Magicka Regeneration" enchantment on an item only takes one Enchantment slot!

The caveat is that they are each weaker than if it had been just "Fortify Destruction" or just "Magicka Regeneration".

With dual enchant you could put "Fortify Destruction & Magicka Regeneration" AND "Fortify Restoration & Magicka Regeneration" on a single item!!!

To answer your question about finding just Fortify Destruction, look for the ring as someone pointed out or I've seen a few helms with it.

Thanks.. if it's one slot then I can't figure out what items will allow me to enchant with it.. I've tried hoods, rings, amulets etc .. it says the enchantment is not valid for that item.