I've read lot of posts and tries many searches but could not land on cheap flights from Toronto to Serbia ( Belgrade or Nis or else ). Challenge is that flights from Toronto canada to first stop in Europe include major airlines and I want to take advantage of low cost airlines like Ryan air that fly from many locations into Timisoara or Nis. Is there a search engine does combines both. Thanks.
To make use of the low cost carriers (like Ryanair) on an itinerary like that, you will have to book a 'self connect' trip. That means that you'll be traveling on multiple bookings. One to get you from Toronto to a major European hub city (and back) and another to get you from that city to Belgrade (and back).
The danger here is that you are entirely responsible for making the connections. You'll need to allow a good amount of time between flights and may find yourself forced to book (and pay for) a replacement flight if an earlier flight is heavily delayed.
Of course, it is possible that the cheapest option is one of the 'regular' itineraries. It depends on the available low cost services to Belgrade.
Obviously you need to get across the Atlantic. Currently the cheapest carrier from Toronto is most likely WOW airlines (through iceland) serving a variety of European cities.
Depending on date and location, this can be less than $400 round trip (no food, no bags, no seat reservation). You can potentially combine one of those with a cheap Ryan Air hop to NIS, which can be as a little as 10 Euro.
You most likely have to budget for a transfer between the two airports (say from Frankfurt to Duesseldorf Weeze) and you will need plenty of time for the transfer.
In contrast, a full round trip on a single ticket to Belgrade will run you around $750. You need to decide whether the savings (after bag fees, transfer cost, etc.) warrant the extra risk and hassle.
Most of the truly low-cost airlines like Ryanair do not allow their prices to be on travel search engines, so you will have to piece together these itineraries manually. Remember that you run the risk of wasting your low-cost connecting flight if your flight into the connecting airport is delayed, whereas if you book this all on one ticket, the ticketing carrier will have a responsibility to get you to your final destination as long as the misconnection was not your fault.