I've found glass-bottled water easy to find in Belgium, I buy cases of Ginstberg (they have both still and carbonated water) in glass bottles in a supermarket here in West Flanders.

I know at least a couple places where you can buy it (Huis Maria in Harelbeke, Vanuxeem in Ploegsteert, so both Flanders and Wallonia) so I would assume those to be widespread enough throughout the country.

To me coming from France it was Belgium that was the easier place to find glass-bottled water :)