said 10 months, 3 weeks ago:
I agree with @charlene-kingston. The best list you have is the opt-in list you built.
You can use tools like blogs, video and social media to increase your online visibility and drive people to a squeeze page where you offer some “email bait” to visitors in trade for them signing up for your email newsletter.
It could be a white paper, store discounts, a giveaway, what have you.
You can also buy lists–which I never recommend–as a way of jump starting your list, but the quality of these lists are suspect IMHO. More expensive lists might be different, but for a startup they may be out of your range.
Pay-per-click ads on Google or Facebook can also drive targeted traffic to a squeeze page for lead capture.
Also, I’ve had luck by putting on free and inexpensive presentations through local groups, like Chamber of Commerce or marketing associations. They’ll often give you attendees emails, or I often will say that I’ll send anyone the slide deck for their business card, and in the follow up suggest they join my list.