From my experiences, trying to catch island beetles after 7pm is pretty hard. Then again, that might be because I have a ton of flowers on my island and a few lemon trees.
Selling a load of perfect fruit in somebody else's town is a pretty good way of earning IGB. And like other people have said, trading your TBT for IGB works too.
Also, do you have any popular villagers in your town that you don't want to keep? If you do, then you could sell/auction them for quite a lot of IGB or TBT if they're in boxes. If the price is in TBT, well, you could just do what I (and other people) said earlier and trade it for IGB.
If you don't have many dreamies (favourite villagers) in your town and are willing to TT, you could set up a cycling thread in the Villager Trading Plaza and cycle through villagers. It's a good way to get IGB/TBT from selling villagers you don't want while making it easier to find the villagers you do want at the same time!