I just replaced mine with an 85W mini ITX supply. It comes with the brick for $40 with shipping. http://www.directron.com/htpc1080ip.html
ITX pinout: http://www.powerstream.com/itx-atx-pinout.htm
I was kind of hoping for something cheaper but flipping through electronics catalogs it seems that 3.3V at a large amperage is kind of rare.
I also put in a case fan. I drilled a couple of holes in each corner of a square to fit the jigsaw blade into it. Then used a nail and hammer to mark where the holes for mounting the fan needed to be drilled. Am running the fan on the 5V so it's very quiet but maybe not effective enough (case fans are normally 12V). Others have suggested using +5 and +12 to get a 7V drop across the fan.
Connect the orange (3.3V) and black (ground) wires from the power supply to the same colors on the connector that you've taken off the dead power supply. Connect the red (5V) supply to yellow on the Roku connector. Connect the green wire to a switch if you want or just wire it to ground (always on). You can always power it off by disconnecting the external brick.
Mine is not professionally done at all. I am a software engineer. For example -- I used packing tape for my insulation. I unwisely left the circuit boards in the unit while metal fragments were flying around with the drill and jigsaw. (A fragment of metal could wedge under a component and short out the board.) Finally I used metal screening for the fan (I left the tape on the sides to try to keep wires from coming off the edge of the screening).
The bundle of wires on the left is just left over from the power supply cable; I thought I might use them for something later (like the fan) so I didn't cut them off all the way.