You are correct, if you have moisture/fog/anything between the two panes then the window's seals are broken.Unfortunately there is no effective fix to this problem, other than replacement.
You may well be able to have the glass only replaced at a good window supplier, or maybe the glass and frame, this would save you some money and a lot of money in labor to have the whole window replaced.
Look in your local phone book for window manufacturers, very often siding wholesalers sell windows as well, these would be a good place to start and see if they can match the existing windows and sell you just the moving window.
Here is Loewen's web address, start with a friendly email to them, they will be able to guide you in the right direction toward a dealer.
Loewen // Design. Create. Inspire. 