Everybody else has given places for finished mirrors. The question that I have is - why not make the mirror also? You can go down to the local window glass place and get 12-25mm thick glass (go for about a 1:8 - 1:10 ratio of diameter of the mirror to the thickness of it) and get pieces of glass. You can then easily make circular blanks out of that with a coring tool and you will also want to get a concrete saw blade (diamond grit saw) and make a curve generating tool with it. At this point, you can give the blanks to the students and have them make the mirror in a few weeks and then do the rest of the telescope in the rest of the classes. You could probably even do the preliminary work of coring and curve generation as the first 2 classes for the year.
FWIW, curve generation takes a lot of time and effort out of the mirror grinding process.
There are a fair number of places on the web that describe the mirror grinding process and also some that describe the coring process and for the curve generating, probably the best is Peter John Smith`s website at http://www.users.bigpond.com/pjifl/ . The last thing required is grits and these are available in a lot of places and you won`t need much of them compared to buying a complete mirror - 1 finished mirror price should be enough to buy the grit for about 100 mirrors with some left over in the smaller sizes. The secret place for the grit is places that sell to the eyeglass industry and you want to buy the grit in 2kg or larger packages as packaging is a large part of the price of the stuff.
Good Luck