I asked around for 19mm plate glass and I was quoted $180 for a 250mm piece. Not all glass shops cut glass 19mm thick but if you ring around you may find some that do. One of the problems with cutting glass is that you may release strains on the glass and that may not allow you to get a good figure. Have a look at this site for plate glass
http://www.galvoptics.fsnet.co.uk/telescope3.htm . I came up into a site in the US who were selling porthole glass but I can't remember the name. I'll keep looking and if i find it I'll let you know.
Regards Cristian