Most folks tend to put them on the passenger side to offset the driver weight. Whether that is the right thing to do or not is going to depend on the particular car, and can only really be confirmed with a set of scales.
Summit sells kits, as do a few othe rplaces, but I'm not a fan of the plastic battery boxes in most kits, and even the metal boxes have thier downsides as well. Most of the kits I've seen tend to use smaller cable than I like too.
My personal preference is to piece together my own bits, an Odyssey battery (PC680MJ or PC925MJ) and one of thier aluminum holddowns, a 0 guage cable for power, and another piece of 0 guage for the ground to the body, a fuse or breaker on the power line, mounted near the battery, that is large enough to handle your starter current, and a good power distribution block up front to tie all the power wires together.