Q: can't find series "dragons - race to the edge"

Not everything is available from iTunes (and some things are available, but not in all countries) - what is available, and where it's available, is controlled by the content providers/rights-holders, they provide the content and grant Apple licenses to sell it on a country-by-country basis

If it's not available in your country's iTunes Store you could try contacting the TV company and ask them let Apple sell it there - you could also try requesting it via iTunes, but Apple won't be able to do anything without the rights-holder's agreement : http://www.apple.com/feedback/itunes.html