After having trouble with my rennet purchased specifically from a Cheesemaking supplier I decided to give a cheap bottle of Renco Rennet a try just to get over the awful curse of no curd!
For $2.20, and purchased at my local supermarket, it worked perfectly! I have had a great curd and break since and I have had no troubles at all with both soft and hard cheeses.
I'm not sure how 'pure' it is in the eyes of professional cheesemakers but it's working for me so I recommend it.