There are many forums and places where Amazon sellers congregate online, if you can find where these audiences congregate then this could be a way to approach them.

If you have found niches is there any reason you wouldn't want to develop an online presence yourself?

Platforms such as Amazon, eBay, Etsy and Shopify make it very easy to get going.

There are even software tools to allow you to manage platforms centrally.

The cost, knowledge required and the barrier to entry is low, and once you can see what is working via the platforms, then selling directly via Shopify or similar can help you get even higher margins

Good luck!