I have 6 through 9 for mine. I've never seen slugs or steel available for 28. And I have looked.....
Go here. They have 28 ga. slugs available for reloaders. They also have .410 slugs. Cool, huh?
I'm a dumbass. Here's the link for those 28 ga. slugs:
http://www.ballisticproducts.com/bpi/1product_articles/components/slug/slugs1.htm