If you need small amounts for your experiment, I might be able to help you out but didn't see your email in the profile.
Homebrew stores usually carry these, so Homebrew Emporium in Cambridge and Beer and Wine Hobby in Woburn are fairly safe bets. I think rennet tablets last a long time, but still worth checking packages for dates (I need to do this for mine too). Liquid rennet needs to be refrigerated and would be more perishable.
I have seen rennet in some Latino markets, but am not certain enough of availability to send you there.
Cheesemaking.com is in MA and pretty quick.