Besides turnips, the main bell earner is perfect fruit if its on premium and it isn't your town perfect fruit. So you need to trade to get foreign perfect fruit.
Other ways - have bell boom on as the ordinance and bug/shark farm at your own island. This is the main way I make my bells.
Minor ways - use the flea market at R-Tail - I make around 40k per character in there. (but I play slow with multiple loads).
Other ways, well it means trading with other players. (eg villager trading).
