FBinNY 

I suspect that your real problem isn't the bolt itself, but what caused the other to break in the first place.

Odds are the post doesn't fit properly and the bolt needs to be over-tightened to hold the post. By now you may have a worse problem in that over tightening may have bent the ears so they now touch in the back, limiting how tight the tube can close.

Start by taking a look at the ears to see if they touch, and also the slot to see how far it closes. Most seat tube slots are about 3mm or 1/8" wide before clamping. If they close so the width at the top is less than half the width at the bottom than odds are the post is too small. If the slot width looks OK but the ears touch, then check for cracking where the ears join the tube, or for bending. Ultimately these problems would need to be fixed before any bolt can solve the slippage problem.

In the meantime, especially if the problem is only marginal, you can improve the grip with lower clamping force by using some coarse lapping compound on the post in lieu of grease. Lapping compound is basically abrasive grit in a grease base, and when compressed between two parts will bite into each creating a strong mechanical interlock that only needs minimal pressure to keep from slipping.