I buy my matte board and foam core (for backing) from a supply distributor in Virginia, to avoid shipping (I drive there once in a while and get 25 sheets of 32x40 of each and cut down). If doing 20x24 (a bit wasteful with the 32x40) I buy precut online, but not windowed.
I get all my frames from Mark (posted above). Frame Destination is reliable and prompt. Good mounting and framing info on their site, too.
It's a learn by doing and observing thing. Practice test cuts with the cutter first. You want to just penetrate the board, no deeper than necessary, and with a backing piece on the other side. Logan customer service is very helpful too, and you can get someone on the phone.