What specifically do you need? There are some posted in MrDS' resource thread before like Mega Altaria, Mega Rayquaza, Mega Aggron, Mega Swampert, etc. but they didn't meet the resource's requirements... Should I compile them for you?
I also did submit some before but mostly front sprites that are not well resized and I don't recommend them (some of my submissions) for now, especially the older ones...