Flying from Bruges, Belgium to Zagreb, Croatia

Bruges doesn't have an airport.....try Brussels, Amsterdam, or Paris instead.

Croatian airlines flies direct Brussels-Zagreb a couple of times a day. If you're only flying one-way, still look at a round-trip ticket....it'll be cheaper than a single-one way...as low as about $200. I notice there are very few cheap seats left, even into October.
If you can't find anything available there, you might want to try exploring a website like whichbudget.com . The best possibility it shows is easyjet Brussels-Milan, then Germanwings Milan-Zagreb. Be aware that any arrangement like this will be two completely separate flights....if the first one is delayed and you miss your second flight, your second ticket is no good. Even if you make a two+legged flight on the same discount airline, they don't have an obligation to put you on another flight.