We do a very similar practical at our school.
We use Bull semen rather than pig (avoiding any religious problems) it is simply to look at the sperm cells under the microscope during the reproduction topic. The students find it quite an interesting exercise, after getting over the usual shrieks of hilarity and disgust!
We have found that UK Sire services are a very co-operative company to do business with. Simply google the company and you will find them pretty quick. Don't worry too much about the rather large quantities of money this premium product seems to sell for to the discerning cattle breeder.
If you phone them up and explain the situation to them - you are a school, you just want to view any old sample under the microscope etc. they should be able accommodate your needs, they have always been very friendly and understanding and have provided us with samples for several years now at a very reasonable cost.
Having done this practical exercise for a few years now I find it best to make sure the teacher controls the sample and dispenses 1 drop per slide as needed (you only get a very small sample) Also only order it in very close to when you want the class to use it, it really has a very short fridge life and does not stay motile for more than a few days.
Happy viewing.