How to get pornographic movies for scientific research purposes?
I really don't get this. This is a little like the Movies Classification Board or the Classification &amp; Ratings Administration feeling awkward about asking to view the movies they are going to rate.

If you are developing an algorithm for detecting pornography, why would it be awkward to request funding to acquire test resources? This is an entirely legitimate application, that has huge potential, both economically and socially (note: use of bold inspired by OP). There are many researchers all over the world working in similar fields. Clearly, at some point you will need to discuss your work with your PI, and presumably, they are already aware of the work you have been doing in the previous 6/9/18 months, and perhaps already requires justification against existing research funding.

Whatever the case, your work will need to be approved by, and comply rigidly with, your institution's ethics committee. 

There are also a number of steps you could consider before committing to viewing full-length extreme hardcore films. In making some suggestions, I'm going to make a few assumptions:


1 Your definition of a pornographic movie is one classified as
"mainly concerned with sex", and involves extreme nudity and acts of
sexual behaviour.
Your algorithm looks at exposed and interacting body content in some way, and does not work on quality of script and acting (otherwise daytime soapies will bring up many false positives)
Your algorithm looks at small fragments of movie clips, and not the entire 60/70/90 minutes worth, or however long these movies go for. 
Your PI knows what you've been spending your previous doing, which is most likely already associated with existing research funding.


So, some suggestions


Acquire a number of movie trailers; these are usually freely available for all types of movies, and typically run for about a minute. Contact the various movie studios directly if you need to.
Speak to your national Film and Television archive body. They are likely to have a number of suitable media to preliminary early test procedures on. Various bodies exist.
Speak to other researchers in this field. There might be a good standard set of images/movies that can be used to determine baseline effectiveness.
Go to your local friendly adult shop and check out the "specials" bin. 
Finally, and not the best approach to take, look at the internet. I'm guessing if you typed in the right combination of words, you'd get unlimited supply of short, copyright-free videos, to use both as a test group and control group. But I stress, this needs to be done in strict accordance with your institutions policies for network and    computing use, and therefore should be approved by your PI and ethics committee.


However, whatever approach you take, the summary is: talk to your PI.