Where and how to get riot prepaid card?

Hi, unfamiliar with how this works.

currently the usual method I use to pay is unavailable, leaving only Prepaid cards as an option, never bought one before. Is there a more direct store, or buying one from Amazon is fine, or even from local shops?

Will any prepaid card of Riot will work? I'm seeing one of each game except LoR. some say 'NA Server Only' i'm from Philippines, will that still work? hope to get some coins handled for the next patch