I live only 20km from the Schwentine river that holds +100lbs Wels catfish. It only takes a week to catch one if you know what you`re doing, i.o.w. bring a few years of experience and renomee in the wels angling community. Down south they have hucho, a fish very similar in size and apearance to that elusive dorado. It only takes a winter to catch one, given you are willing to afford a 4 digit license fee and dozens of snagged lures. If I combine that with a few million very poor people unafraid to use nets, snag lines, poison etc. and the reputation of the south american law enforcement community.... I once been to Bulgaria and again, my travel guide said she was into fishing but never went for it because her countrymen had devastated the stocks in fresh water.
What makes you thing you can just go out and catch one like you do going after stocked wipers? My guess is that it will take a very unaffordable, rather unpleasant trip into very remote areas with a guide and everything.