You can get them on ebay.
There are 3 parts you need, the digitizer\front glass (all one unit), the bezel (this will stretch and most likely break when you remove the broken part so can not be reused) and the home button frame (you need to recycle your current home button component which is a bit fiddly as it needs to be cut out and then remounted into the new components during assembly).
Although a little more expensive than buying the parts separately it is a whole lot easier to get a complete pre-assembled unit which includes a whole new fitted home button component, that way all you need to do is remove the old part, connect one ribbon cable and snap the whole unit back into place.
You can pick the complete unit up for under $30 shipped
HERE. I have used this guy several times for parts and never had any problems with any of the components. From memory I think he also includes the plastic tools need to remove the old part but check with him to make sure.
I have done a couple of broken digitizer replacements and on one of them I managed to crack the LCD whilst removing the broken digitizer so be as gentle as you can and work slowly trying not to apply to much downward pressure. Also check out youtube and ifixit.com for disassembly guides.
Edit: If your touch is the latest 8GB model then you need the touch 2G parts. Also try not to touch the LCD as it is a pain to remove any smudges and can introduce dust and lint so make sure to work in a clean area and don't remove the protective film from the inside of the digitizer until you are about to snap the unit into place. There is nothing worse than fitting it all back together and turning it on to find there is dust under the screen.