maybe the cracked ones are the old ones that were sitting around.
77 previously did not stock sava's , they outsourced em and had em drop shipped to you. They never saw the tires. So it would not be thier fault nor would they know if you got a crappy old set. I know they had problems with distributors.
MWH for good or for bad has sava's in sets with tubes, and every time ive ordered them , they have quickly sent me new tires and tubes for a decent price.
A search for moped tires under ebay brings up some too.
try denniskirk.com for irc's or quarterkick like others say if you want to try the irc's. I like them alot too.