nearly all flasks are 'all stainless' these days. IME they are not quite as effective as glass Dewar flasks but they are considerably more robust.
Cheapest option (last time I looked, about a year ago) for something that you didn't buy sight unseen was in Tesco's; they do/did a small stainless flask that ought to fit a bottle cage (check measurements carefully) for about four quid.
cheers