The easiest way to get bells in the game is to visit the Island Resort after nightfall. When you leave the hut, you can talk to the gyroid and he'll let you borrow any tools that you need. Using the net, you can capture beetles that appear on trees there. While you are on the island, you can gather more insects than you would usually be able to carry, then take them back to the mainland and make several trips to the store. The golden stag beetle is worth 12,000 bells at Re-Tail, and other insects fetch a similar price. If you want to improve the rate at which valuable insects spawn, you can chop down some of the trees at the center of the area and dig out their stumps.