I looked around on this a little at work to see what people are using. Came up with a couple possible ideas if you haven't bought anything yet:
Home Depot Empire levels and square are warranteed for life against materials/workmanship. Have you tried swapping it if you bought from there?
For the 12" the starrett heads fit just fine on empire rulers. Not sure on the 6", but amazon sells the 6" starrett head for only $30. I ended up having a 16" empire square too.
I'm just hard on these things I guess though. If it weren't for my company buying them I wouldn't spring for starretts at work, neither brand seems to last me that long.