If you want a cheap flight from Doncaster to Sapporo, you've got zero airlines to choose from.
Make sure you use the search options above to search for flights to other airports in Sapporo, not just New Sapporo International Airport. The more options you look at, the better your chance of finding a cheap deal.
During this peak season, accommodation prices often increase too, so it's best to book early. Or you could keep your fingers crossed and hold out for a last-minute deal.