SanDisk 4GB Ultra II Memory Stick PRO Duo For $89.99 After Rebate @ Frys
Right now Frys has the SanDisk 4GB Ultra II Memory Stick PRO Duo for $119.99 with a $30 mail in rebate, bringing the final price to $89.99. As of the time of this post they are in stock at Frys.com but I'm sure you can also find them at your local Frys store.
Check it out here.Via PSPUpdates