Depending on your region in the world, they are available online.
Where the heck are you going to find blank 100 gig disks?
Those are also available [but expensive] when shopping online, assuming you're in a region which sells them.
It took an hour to burn a 25 gig Bluray disk
Unless you're using 2x media (rewritable?), it no longer takes that long.
so a 100 gig disk would take 4 hours to burn a movie that can be watched in real time in 3 hours.
At 4x, a 100 GB disc takes 1.5 hours or so to burn. 2x would be 3 hours, or roughly realtime.
Now, with verification enabled (where the drive actively checks the written data throughout the burn, not the post-burn verification), the time is significantly extended. But such in-burn verification is an option.
EDIT: But I get the point being made. This stuff really is inconvenient, and in order to have something of quality, it's very expensive and time consuming.