Not all of the G30 series boards are good overclockers - just certain ones do. Asus did a couple of good G33/G35 boards, as well as the Gigabyte. I think it was just a case of there not being a good overclocking LGA775 mATX board on the market and certain companies wanted to see if they could make some money from one.
As for finding one new, well, you won't. Best place to try is the second hand places really.
The P45 mATX boards are a lot more expensive because you're paying for a faster, more overclockable chipset, better southbridge, and Crossfire. They're not really aimed at the same market segment as the majority of the G30/40 boards, which is budget HTPC.