There should be a grommet in the firewall somewhere... I have yet to come across a Honda without an unused grommet somewhere in the firewall. It should be pretty easy to find from the engine compartment... you can use that grommet, just poke a hole in it with an awl or similar tool and push the wire through it... the rubber should be tight against the wire insulation to prevent water/air from getting in. It is a good idea to run the wire in split loom tubing, this not only gives a factory appearance but protects the wire from chaffing, oil and heat. Be sure to tie the wire up well to prevent any interference with the cars operation. You should be able to find 1/4" or 1/8" loom at a home center or electrical supply store.

www.parts-express.com will also have everything you'd need to get going... the dash piece, harness, loom, wire... everything. But sometimes its just more convient to buy things like that locally.

With your car, you can re-use the stock mounting brackets to mount the new radio, and use a strap of some-sort to attach the pocket to the radio... this would eliminate the need for a dash kit, but leaves a tiny gap around the radio and is a little more time consuming to do. For ease of instalation, a dash kit maybe your best bet.

good luck