Here is a discussion about grounding locations that was posted yesterday.

Yes, there are different gauges of ground cables. I would not buy one at a swap meet unless the gauge was listed on a packaging label. Most of them would most likely be 6 gauge (or 4 gauge at best). You can get braided ground cables down to 2/0 (00) gauge.

Tractor Supply Corp. has 2 gauge and 4 gauge braided ground cables (battery to frame), but they are not very long.