Either NTF communications in the northeast or Dean at Comtek (at the Road King on Barlow) will sell you exactly what you need for coax.
As far as increasing the guage of the wire, bear in mind that the maximum transmit power of your CB is only 4 watts. Assuming that your radio is only 50% efficent (a figure that I pulled out of nowhere, but one that'll work just fine for our purposes here), that still means that the power requirement of your radio is likely to be somewhere around 8 watts, and the actual current draw of your radio is likely to be 0.6 amps. Probably not enough of a power requirement to be worried about at this point in time. Your instinct is correct though - connecting directly to the battery will probably get you cleaner power than splicing into the wiring harness elsewhere. Just be aware that it is still a vehicle, and many a HAM operator has spent months of time trying to chase down electrical noise and eliminate it with mixed success. You may be happier by just pulling halfway clean power off the battery and just saying "It's a trail CB...it's not like I want to talk to Puerto Rico on this thing!"
If you do go with the front fender mount (which does look pretty sharp, BTW), you will want to make sure that you're running a pretty flexible antenna and probably a spring at the base too. I know more than one or two guys that have tried something similar and ended up with a broken antenna due to tree limbs and brush.