The cooking one is available now for 10 days at any DS download station ... in Japan (check nintendo.co.jp )
the 3 other one are now unavailable and currently nobody managed to capture these demos and made them available on the net.
if you have a nintendo wifi USB , you could beam the available E3 2005 demos, DS download Vol1, and some japanese demos to your classic DS. http://simon.teunvandeberg.nl/nifi/howto.html
it's more simplier if you have a PCI/PCMCIA (not usb) RT2500 wifi card since there is a modified windows driver for them.
It's like having a download station at home