http://www.aquariumguys.com/mastertestkit.html
Cheapest I see in the US; we get screwed in Canada though - cheapest I have seen is 36 bucks plus tax.
$36 isn't much. If you feel you're getting screwed, buy it at petsmart and return it when it's half-used. Their return policy seems to be "If we stock it, stocked it, or might stock it, we'll return it for you, receipt or not receipt." I returned a mostly-empty bottle of stress-zyme after finding these boards and realizing it probably wasn't doing anything for me.