can you feed microworms to adult goldfish?

Nope. They're very very small. I would think it's already hard to feed MW to baby gfs.

Where can you get microworms from?

You might find them in independent stores, where somebody raises them. I never had any luck finding those in chain stores. Or you can order them online at Aquaticeco.com. They ship them overnight, and do a fine job at packaging them and all.

You might be able to work them into homemade gel food, but I am just not sure if that is worth all the effort, and did I mention, smell?