It would help to say where you are located, transport costs will be high.

Most towns have glass merchants who can supply picture (float) glass but be aware this is now 2mm in most locations, for Health and Safety reasons 1.5mm is no longer in general use. My frame maker gets much of his glass direct from the suppliers ready cut.

You need to check what thickness your plate holders will accept, if you want thinner than 2mm then it's likely you'll have to go to a more specialist scientific/optical glass supplier.