There are groups on Facebook, Discord, etc. where people are posting about how much Daisy is selling turnips on their island, and how much the Nooklings are buying turnips on their island.
The best way is to start with about 400,000 bells, have an empty inventory, look for someone selling turnips for 95 or fewer bells per turnip and buy 4,000 turnips, so you have 10 bundles of 10 in each slot. Then look for a person with Nooklings buying turnips for at least 500 bells per turnip.
You can make around 2 million Animal Crossing Bells in one go. If you have an agreement or if your island is set on an early morning Sunday, it’ll go quicker. You can make 10 million in a few trips.