I've found it difficult to find beta users through online communities and Twitter. What other channels should I be utilizing?
My experience with online communities is mixed. Those work best if you have a value proposition that appeals to a large mass for a short period of time - i.e. a game.
For something like healthcare I'd imagine you are looking to have people engage regularly and for an extended period of time. That is going to take more time to convince people to use your platform.
You should be looking for your core user. If you developed personas and user stories, you can use those to identify groups to start with and expand from there. A beta test with a few hundred core users is better than 1000s of users that don't fit your model. If you didn't develop personas or user stories, I'd recommend taking time to go back and think through those. Either way, I'd be happy to help you get started.