I've been very happy ordering from Tape Resources: http://www.taperesources.com/

I get my SONY Professionals there for under $3.50 a pop.

Good luck,

There are places where you can get mini-DV tape around $2.60USD/tape.

http://www.dvinfo.net/conf/showthread.php?s=&threadid=37224

There's also the sponsors for this site:
B&H: http://www.bhphotovideo.com/?BI=155 (this might be the referrer link, which helps this site if you do buy)
Express Video Supply: http://www.evsonline.com/ (50+: $2.89 + shipping + applicable taxes)
Zotz Digital: http://www.zotzdigital.com/
Pro Tape: http://www.pro-tape/

CAVEAT: This information applies to US Only!!! Some stores like tapestockonline.com which ship UPS Ground, which is very bad for non-US people. The hidden brokerage fees kill you. Some eBay vendors are excellent for non-US people... but you have to ask them about customs and brokerage fees and such.