Finding an audience prior to launch: Printing

I'm in the process of coding a printing web app which will provide a super easy-to-use interface for graphic designers to print their work on T-shirts, tote bags, posters, phone cases and so on. There's no minimum quantity and customers can provide their own label designs so this could be used as a dropshipping resource for designers.

Obviously the app could be used by anyone but I feel niching down to graphic designers is a good direction to take the initial launch in.

I've heard on various podcasts about getting yourself an unfair advantage before your product is launched by having an audience.

Would it be better to go to where these users hang out - in which case, where might that be? Maybe Dribbble or similar?

OR

Trying to build a mailing list or Twitter following somehow - offering some kind of value in return for an email address or Twitter follow?

If I only had time to tackle one of these, which would be better?

Thanks!