An AM3/AM3+ mother does not support AM2 CPUs unfortunately. The CPU actually doesn't fit into the to a subtitle change in key position and if you manage to get in into the socket it won't either because AM3 motherboards require a processor with DDR3 support and.
So, you need to look for a AM2 motherboard. Most of the Asus M2A-series motherboard are quite descent but Gigabyte and some other brand make nice boards as well. Some computers might still have some in stock but I guess your best bet is Ebay. Just had a wee look for you on Egg and found this boardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813157274,
it wouldn't be my choice though but according to ASRock your CPU is compatiblehttp://www.asrock.com/mb/AMD/A785GM-LE128M/?cat=CPU).
Check your CPU revision though.