The market for right-backs in Europe is well-studied by Barcelona's scouts. The main handicap is that that many of the targets are bound to their current teams, who do not want to let them go.
Barcelona tried to get Hector Bellerin, who renewed his deal with Arsenal. There are other options in the Premier League too, with the club following Chelsea's Victor Moses and Southampton's Cedric Soares.
The latter was on the agenda a couple of years ago when Barcelona signed Aleix Vidal, back when he played at Sporting Gijon. Moses is a newer option who only came onto the radar when Antonio Conte converted him into a wing-back.
Other options are Mario Fernandes, the Brazilian at CSKA Moscow, fast and good technically. Then in Serie A there are Roma's Bruno Peres, and in France Sidibe at Monaco. Barcelona prefer Cancelo though, for his experience of the Spanish league and big potential, but all depends on the competition for him - with Chelsea and Bayern Munich sniffing around.