How do I find my potential customers (B2B business)?

I'm in the middle of building a useful (as I think) SaaS for small e-commerce stores. Before I got too deep into that, I'd like to find out if they are really interested.
In other words, I want to try to sell the product before I finish that.

How do I approach my potential? Should I write tons of (e)mails? I wanted to find about 100 customers in the area where I live (Boston, MA). It will be easier for me to go and sell my product directly to them, not over the phone. I tried to find e-shops using Google Maps, but I found only 20 of them in Boston my area. Yellow pages did not help at all.

Does it mean that the market is just too small or am I doing something wrong?

You might want to start networking with those who sell Merchant Accounts. Braintree merchants is in your area and has a great reputation for E-comm. They will be able to share leads with you. Other than that, get a list of local zip codes and start doing some searches in bing for shops with the zip code.

Its all about networking at this point. Spread the word about your product.
And dont be afriad to look beyond Boston.

If you have 100 leads in Las Vegas (vegas is popular for serious e-comm), then its worth your time to take a flight out to meet with some potential clients.

If your product is pretty niche you could put up some Google Adwords to see if people are searching for something like your product. Are their prospects already in 'pain' and looking for what you have to provide. Setup a basic web page, that says something like Private Beta, and give them the option to add their email to get notified when it is released to the public. This might give you some insight to potential demand.

If it's something new that no one would even consider searching for you will probably have to hit the streets and get in front of people with a prototype and get feedback from them. If they say it's a brilliant idea get a check from them ;)

So you've clearly found some ways in which e-commerce can work better for small operations. And you want to focus on prospective customers in your area.

In my view you could do a lot worse than going to local specialist retailers, and to people selling through markets (Faneuil Hall springs to mind) and ask them what they think of e-commerce, what (if anything) they're doing at the moment, what they'd like to do. If you're respectful and aware of busy times, in my experience people open up naturally.

Be open. You're developing a system for small traders, and you want to make sure it really does a great job - that it catches everything that's important to them to make a difference to their business. Start the conversation, and if you put in the time and shoeleather you'll certainly find 100 contacts, some of whom could be future customers.

Now, plenty of your prospective customers aren't as visible and available - there's no storefront or market day. Their needs will be similar in many respects, different in others. But they're also harder to connect with - as you've found. For me, I'd put this group on the back-burner. Come back to them when you've tested and refined your ideas with visible retailers.

You're ready to go out and sell - so the first step is to go out and ask!