Find target addressable market

We've been building Supportress, a simple helpdesk for remote distributed teams. It's based on my 9 year experience working at GitHub Support. Our vision is to have a bootstrapped and calm company. However, after a year we are at a crossroad. We either find day jobs to keep on bootstrapping or go down the VC/Angel path.

The latter usually involves questions like total addressable market, competition share of addressable market etc.

Could you help me figure this out? What's the best way, site, or service that can help me find out those numbers?