So. Get yourself grape juice (need to get it stale - you can either leave it in the press for an hour or so, or just put it into the bucket, and let it be for the same period. Pour it into a bucket, and leave in a cupboard for a week(ish). It will turn into vinegar. You won't get discovery if you will just lift the bucket, you need to pour vinegar to the other bucket to discover it. Then, you have to butcher a calf or a lamb. During butchering, you will discover 'Suckling's maw'.
Note that you won't need a jar anymore. Get vinegar and the maw with you to a boiling cauldron, wielding an empty bucket. Done, you've got the rennet