I've built a few niche products and managed to try enough stuff to get some decent traction with two of them: notarycrm.com and mailfinch.com

tl;dr - if you're selling B2B, you're not going to get anywhere unless you can reasonably convince someone that you'll save them time, money or (ideally) both.

In general, I spent a lot of time cold calling the first users and gave away the product. There was a lot of back-and-forth about what sucked, what I had recently fixed, etc.

In both cases, the first users helped me understand quite a bit about how I was going to have to scale the sales operations... for example:

With MailFinch, one of my original assumptions was that real estate agents would want to use it to send out all the weekly flyers that they usually send out. So, I first asked all of my friends in the DC area to send me their local newspapers -- I spent a weekend circling all of the real estate ads that were larger than 1/2" (if they're advertising in the newspaper, you can reasonably assume they have money and have some understanding of marketing). I then cold-called the hell out of these people and learned two things:

1. I was right -- they send an average of ~100 flyers each week.
2. They would usually hand me off to their assistant (!!!)

That second item was important -- I didn't need to sell to the agent directly, I needed to call their office and talk to the assistant or office manager instead. I talk about this in detail in my Mixergy interview last year (which is now behind a paywall): http://mixergy.com/mailfinch-paul-singh-interview/

With NotaryCRM, I actually convinced one notary (my mom) to start using it for her side work. Then got my wife to use it. Then one local notary that I cold called. From there, I learned pretty quick that notaries talk... a lot. If I had a bug, everyone started bitching simultaneously. As the product got better/prettier/more useful, everyone started loving it simultaneously.

The big breakthrough was realizing that there are two good channels to reach this market:

1. Cold call the notaries (there are huge directories for this now) and convince them to signup for a free account -- this is generally hard, but I did a lot of it early on.
1a. Make sure you ask for referrals on each call, email, meeting, whatever -- "do you know any other notaries that might find this useful?" Notaries are regular people. And regular people want to seem "in the know" with their friends. If they loved NotaryCRM and referred it to their friends, they generally looked good. Totally serious about this. :)
2. SEO works... especially in niches like this. Checkout notarycrm.com and click on the "Find a Notary" link. I could probably write a huge blogpost about this but the gist is that notaries are searching for their own name -- all the damn time. It also helps that they usually suck at SEO... so it didn't take long for me to put together a directory that came up at the top of the SERPs for their own name. Once they click through, there is a big call to action that says "claim this account" if they haven't already. Again, lots more I can say about this but it might be offtopic from your original question.

So... I hope that helps. :)