dude, you really need to go to a head shop, cuz there are atleast 3 diffrent sized stems/bowls... unless you know the exact measurement, you have a 2/3 chance you're getting the wrong size, not very good odds.. i learned that the hard way... and they all LOOK the same size, but when you try to put the stem in the bong, or the bowl in the stem, it just won't go... so you wasted money...

take it to ahead shop, take either your bowl or your broken stem to them... if you can't go to a head shop, then i would find a site that sells them in all three sizes and buy one of each, so you know what size you're getting.