It can be a bit of a 'how long is a piece of string' question!
Is it just to hold the pistol, or do you want to carry other stuff with it, targets, tools etc?
Will you carry all the other bits in another case or bag?
What sort of budget, do you want a hard Peli case, (or cheaper look-alike) or a softer padded bag?
The questions could go on and on! As a starting point, Solent plastics do a good range of generic hard cases at a range of prices.
Go and have a head scratch...