Well there three tiers of ISPs, each one buying service from the one above them. It's ISPs all the way down, and the higher up you go the more expensive the hardware to run it gets.
At the T1 level it's more completely a mesh type setup, but even lower tier ISPs might set up peering agreements to bypass their main higher tier ISP where it makes sense for cost or service quality reasons. Or refuse to to extract more money as in the comcast vs level1 disputes over netflix traffic a while back
gns3 perhaps? I haven't had that setup for a while but I loved it. I had my whole small ISP in it at some point to work as a test/lab env for testing things out. It's a trick to get going but was kinda fun. I took a copy of that when I left and every now and then I fire it up and mess around with my old dsl/dialup ISP from back in the day.