The link takes you to the complete holder and flask but I contacted them and asked if they sold the flask alone. They did and it cost £170 for a glass one or if you want the pewter version then it is £75.
I couriered my flask holder to them so that they could fit the flask to the holder. (It is well worth sending the holder for them to fit the flask)....I had already bought a metal flask elsewhere but had to return it as it didn't fit!
The glass is thickish so it is very sturdy......I am hoping that I don't have to test this though!!!!!!
Will make a flask to suit your case, it takes a couple of weeks but well worth it, they use a high lead content glass and blow by hand so you get a really solid flask. plenty of pictures on the website.