My a-string broke at the saddle so I figure there must be a bur or something, so I was going to use some emery cloth in there to solve that.

The problem is that what I picked out looks too abrasive.

So I was wondering where would be the best place to get a good emery cloth specifically for this reason (I went to a hardware store and they didnt know what the hell I was talking about). Also, if I decide to use a fine sand paper, what grit would be suitable?

HIgh grit - in the thousands. YOu don't want to take too much off and unbalance the bridge, so a fine grit would do well. It'll take a bit more time, but it'll be done right and lessen greatly the chance of you messing up and shaving too much off.

Try a different set of strings though, man. The saddles are a high stress point for the strings, and sometimes you just get a ****ty set.
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Cool, thanks.

So with sandpaper in the thousands range, should I make just like 3 passes through the saddle?

How can I tell how much is enough so I don't screw up?