I started out using liquid rennet I purchased online and when I got low and found it locally, I went to using Junket rennet. It worked just fine for me, but that said, I am glad that I started out with "real" cheese rennet so that I knew when I needed to adjust the amount of Junket because it isn't as strong.
Walmart quit carrying it here when they remodelled, so I bought powdered calf rennet online. The nice thing about it is that it doesn't deteriorate in strength, as liquid rennet does.