i *think* the best place for diamond hole saws is ebay...otherwise they are super expensive...and dont last very long.
i prefer to get my cheap bulkheads from premiumaquatics....then i can get the really beefy/cool ones from mcmastercarr (plastics company online...google them).
if you would rather go the dremeling route for making a hole for your bulkhead, instead of getting those itty bitty diamond bits lowes sells...i'd suggest ordering these since they are much larger and easier to use: http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=36252