Always start with the destination country's own customs authority website; it's the only source that's both current and authoritative for duty-free allowances, restricted item lists, currency reporting thresholds, and declaration procedures. Most customs agencies have an English version of at least the traveler section, and many now publish official apps that include declaration forms and searchable item lists.
A second good layer is your own government's travel advisory site, which summarizes entry rules for its citizens, and the destination's embassy or consulate website, which often has plain-language guides. Airlines sometimes summarize the rules too, but treat that as a starting point, not gospel.
Be careful with third-party sites that look official but exist to sell forms or visa services that are free from the government, and with old forum posts, since allowances and food rules change frequently. If a specific item worries you, like a medication or an unusual food, emailing the customs authority or the embassy before travel gets you an answer you can save and show on arrival.