- register a legal entity (coop, non-proft, corp, whatever)
- register with IANA to get an ipv4 and ipv6 block
- find an ISP who can do the BGP announcements for you (DSL resellers usually support it, even if for a residential connection)
- use something like Quagga for routing, no need for big expensive routers
- connect with whatever premises you want your DYI-ISP to cover: ex: specific building, such as a student dorm, specific residential district with a mesh wifi, cat5 flying all over the place, install your own fiber (it's not that complicated!), etc.
Browsing the FFDN database, a lot of their members are small orgs with 50-100 users. With time, if you encourage a community legal model (coop or non-profit), people will develop more expertise and step up to manage and expand your network.
PS: I'm part of a Montreal mesh community called "Réseau libre" (http://www.reseaulibre.ca). We don't provide Internet, but we build the underlying infrastructure. It's a lot of fun and a great way to learn crazy networking stuff on a shoe-string budget.