You can try Canada Computers at Pacific Mall in Markham. They have really good prices overall, and they sell the 256Mb SanDisk SD card for CDN$132. Their website is
www.canadacomputers.com.
You can also try out
www.cendirect.com. They seem to have a good selection of memory cards for a really reasonable price. It should also be noted that FutureShop cut flash memory prices as well.
Cheers!