As of right now you cant get a guitar hero 3 guitar on its own. there are a bunch of guitar hero 2 ones you can get if youre only playing guitar hero 2 (those cant be used in gh3) but seeing that youre playing on the wii.. i think youre out of luck on this one.
the DDR mats can be found at various stores but i cant name any off the top of my head. If you really wanted to be hardcore with it i remember futureshop selling the metal ones and i think they could be used on any system.. but dont take my word for that one.