After all these years, Mega Man’s finally getting his mug on various articles of clothing...
Capcom has been selling various tee-shirts of various styles, and the number is always growing. There are also some third-party stores that offer them.
There are a couple of official Mega Man Halloween costumes, though I’ve never been fortunate enough to find a local store selling them. You can probably find them online. See the Buyer’s Guide. In addition, Capcom has begun selling individual costume pieces such as Mega Man’s helmet (light-up!) and arm cannon and such.
This category is really just starting to take off as of this writing, so I haven’t yet fully documented everything. I’ve seen many more tee-shirts as well as sweatshirts, and there have been mentions of pajamas and things though I’ve not seen those in stores yet. Still, keep your eyes open. Most of the apparel right now is “NT Warrior” based.